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Published on: 25/10/2025
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3 Marks
1.
Suppose we have four boxes A,B,C and D containing coloured marbles as given below:
| Box | Marble colour | ||
| Red | White | Black | |
| A | 1 | 6 | 3 |
| B | 6 | 2 | 2 |
| C | 8 | 1 | 1 |
| D | 0 | 6 | 4 |
One of the boxes has been selected at random and a single marble is drawn from it. If the marble is red, what is the probability that it was drawn from box A?, box B?, box C?
2.
Find mean(\(\mu\)) and variance (\(\sigma^2\)) for the following probability distribution:
| X | 0 | 1 | 2 | 3 |
| p(x) | 1/8 | 3/8 | 3/8 | 1/8 |
3.
A pair of dice is thrown 4 times. If getting a doublet is considered a success, find the probability distribution of the number of success.
4.
In a factory which manufactures bolts, machines A, B and C manufacture respectively 25%, 35% and 40% of the bolts. Of their outputs, 5, 4 and 2 percent are respectively defective bolts. A bolt is drawn at random from the product and is found to be defective. What is the probability that it is manufactured by the machine B?
5.
A and B throw a pair of die turn by turn. The first to throw 9 is awarded a prize. If A starts the game, show that the probability of A getting the prize is \(9\over17 \)
6.
Two groups are competing for the position of the Board of directors of a corporation. The probabilities that the first and the second groups will win are 0.6 and 0.4 respectively. Further, if the first group wins, the probability of introducing a new product is 0.7 and the corresponding probability is 0.3 if the second group wins. Find the probability that the new product introduced was by the second group.
7.
There are three coins. One is a two headed coin (having head on both faces), another is a biased coin that comes up tails 25% of the times and the third is an unbiased coin. One of the three coins is chosen at random and tossed, it shows head, what is the probability that it was from the two headed coin?
8.
A card from a pack of 52 cards is lost. From the remaining cards of the pack, two cards are drawn at random and are found to both diamonds. Find the probability of the lost card being a diamond.
9.
An experiment succeeds twice as often as it fails. Find the probability that in the next six trails, there will be at least 4 successes.
10.
Suppose that 5% of men and 0.25% of women have grey hair. A grey haired person is selected at random. What is the probability of this person being male? Assume that there are equal number of males and females.
11.
Of the students in a college, it is known that 60% reside in hostel and 40% are day scholars (not residing in hostel). Previous year results report that 30% of all students who reside in hostel attain A grade and 20% of day scholars attain A grade in their annual examination. At the end of the year, one student is chosen at random from the college and he has an A grade, what is the probability that the student is a hostlier?
12.
Bag I contains 3 red and 4 black balls and Bag II contains 4 red and 5 black balls. One ball is transferred from Bag I to Bag II and then a ball is drawn from Bag II. The ball so drawn is found to be red in colour. Find the probability that the transferred ball is black.
13.
Suppose a girl throws a die. If she gets a 5 or 6, she tosses a coin three times and notes the number of heads. If she gets 1, 2, 3 or 4, she tosses a coin once and notes whether a head or tail is obtained. If she obtained exactly one head, what is the probability that she threw 1, 2, 3 or 4 with the die?
14.
Ten cards numbered 1 to 10 are placed in a box, mixed up thoroughly and then one card is drawn randomly. If it is known that the number on the drawn card is more than 3, what is the probability that it is an even number?
15.
A fair die is rolled. Consider events E = {1, 3, 5}, F = {2, 3} and G = {2, 3, 4, 5} Find
(i) P(E|F) and P(F|E)
(ii) P(E|G) and P(G|E)
(iii) P((E ∪ F)|G) and P((E ∩ F)|G)
16.
Events A and B are such that P(A) = \(1\over2\) ,P(B) = \(7\over12\) and P(not A or not B) = \(1\over4\) State whether A and B are independent?
17.
Suppose X has a binomial distribution B(6, \(1\over2\)). Show that X = 3 is the most likely outcome.
18.
Mother, father and son line up at random for a family picture
E : son on one end, F : father in middle
19.
Let A and B be independent events with P(A) = 0.3 and P(B) = 0.4. Find
(i) \(P(A\cap B)\)
(ii) \(P(A\cup B)\)
(iii) P(A|B)
(iv) P(B|A)
20.
A die is thrown three times. Events A and B are defined as below:
A: 4 on the third throw
B: 6 on the first and 5 on the second throw.
Find the probability of A,given that B has already occured.
3 Marks
1.
Let be the event of drawing the red marble.
Let , and respectively denote the events of selecting the box , and .
Total number of marbles
Number of red marbles
Probability of drawing the red marble from box is given by .
\(\therefore P\left(E_A \mid R\right)=\frac{P\left(E_A \cap R\right)}{P(R)}=\frac{\frac{1}{40}}{\frac{3}{8}}=\frac{1}{15}\)
Probability that the red marble is from box is .
\(\Rightarrow P\left(E_B \mid R\right)=\frac{P\left(E_B \cap R\right)}{P(R)}=\frac{\frac{6}{40}}{\frac{3}{8}}=\frac{2}{5}\)
Probability that the red marble is from box is .
\(\Rightarrow P\left(E_C \mid R\right)=\frac{P\left(E_C \cap R\right)}{P(R)}=\frac{\frac{8}{40}}{\frac{3}{8}}=\frac{8}{15}=0.53\)
2.
\(\begin{array}{|c|c|c|c|c|c|} \hline X & 0 & 1 & 2 & 3 & \text { Total } \\ \hline P(X) & \frac{1}{8} & \frac{3}{8} & \frac{3}{8} & \frac{1}{8} & 1 \\ \hline X P(X) & 0 & \frac{3}{8} & \frac{6}{8} & \frac{3}{8} & \frac{12}{8}=\frac{3}{2} \\ \hline X^{2} P(X) & 0 & \frac{3}{8} & \frac{12}{8} & \frac{9}{8} & 3 \\ \hline \end{array}\)
\(\operatorname{Mean}(\mu)=\Sigma X P(X)=\frac{3}{2}\)
\(\text {Variance }\left(\sigma^{2}\right)-\Sigma X^{2} P(X) \quad \mu^{2}=3 \quad \frac{9}{4}=\begin{aligned}\frac{3}{4} \end{aligned}\)
3.
| X | 0 | 1 | 2 | 3 | 4 |
| P(x) | 625/1296 | 500/1296 | 150/1296 | 20/1296 | 1/1296 |
4.
Let events B1, B2, B3 be the following :
B1 : the bolt is manufactured by machine A
B2 : the bolt is manufactured by machine B
B3 : the bolt is manufactured by machine C
Clearly, B1, B2, B3 are mutually exclusive and exhaustive events and hence, they represent a partition of the sample space.
Let the event E be ‘the bolt is defective’.
The event E occurs with B1 or with B2 or with B3. Given that,
P(B1) = 25% = 0.25, P (B2) = 0.35 and P(B3) = 0.40
Again P(E|B1) = Probability that the bolt drawn is defective given that it is manufactured by machine A = 5% = 0.05
Similarly, P(E|B2) = 0.04, P(E|B3) = 0.02.
Hence, by Bayes' Theorem, we have
\(\mathrm{P}\left(\mathrm{B}_{2} \mid \mathrm{E}\right) =\frac{\mathrm{P}\left(\mathrm{B}_{2}\right) \mathrm{P}\left(\mathrm{E} \mid \mathrm{B}_{2}\right)}{\mathrm{P}\left(\mathrm{B}_{1}\right) \mathrm{P}\left(\mathrm{E} \mid \mathrm{B}_{1}\right)+\mathrm{P}\left(\mathrm{B}_{2}\right) \mathrm{P}\left(\mathrm{E} \mid \mathrm{B}_{2}\right)+\mathrm{P}\left(\mathrm{B}_{3}\right) \mathrm{P}\left(\mathrm{E} \mid \mathrm{B}_{3}\right)} \)
\(=\frac{0.35 \times 0.04}{0.25 \times 0.05+0.35 \times 0.04+0.40 \times 0.02} \)
\(=\frac{0.0140}{0.0345}=\frac{28}{69} \)
5.
S: getting a total of = {(3, 6), (4, 5), (5, 4), (6, 3)}
\(P(S)=\frac{4}{36}=\frac{1}{9} \cdot P(\bar{S})=\frac{8}{9}\)
A can win in 1st, 3rd, 5th, 7th, ..... throws
\(P(A) =P(S)+[P(\bar{S})]^{2} P(S)+[P(\bar{S})]^{4} P(S)+\cdots \cdot \)
\(=\frac{1}{9}+\left(\frac{8}{9}\right)^{2} \cdot \frac{1}{9}+\left(\frac{8}{9}\right)^{4} \cdot \frac{1}{9}+\ldots \ldots \)
\(=\frac{\frac{1}{9}}{1-\frac{64}{81}}=\frac{9}{17} \quad\left[\begin{array}{l} \text { sum of infinite GP } \\ a+a r+a r^{2}+\ldots=\frac{a}{1-r} \end{array}\right]\)
6.
Let E1 and E2 denote the events that first and second group will win. Then,
P(E1) = 0.6 and P(E2) = 0.4
Let E be the event of introducing the new product.
Then, \(P\left(\frac{E}{E_1}\right)=0.7 \text { and } P\left(\frac{E}{E_2}\right)=0.3\)
Now, we have to find the probability that new product is introduced by second event.
\(\therefore P\left(\frac{E_2}{E}\right)=\frac{P\left(E_2\right) P\left(\frac{E}{E_2}\right)}{P\left(E_1\right) P\left(\frac{E}{E_1}\right)+P\left(E_2\right) P\left(\frac{E}{E_2}\right)}\)
\(=\frac{0.4 \times 0.3}{0.6 \times 0.7+0.4 \times 0.3}=\frac{0.12}{0.42+0.12}=\frac{0.12}{0.54}\)
= 0.22
7.
Let the events Be:
E1 : coin is two headed
E2 : coin is biased(heads 75%)
E3 : Coin is biased(tails 40%)
and A : coin shows up head
\(P({ E }_{ 1 })=P({ E }_{ 2 })=P({ E }_{ 3 })=\frac { 1 }{ 3 } \)
\(P(A/{ E }_{ 1 })=1,P(A/{ E }_{ 2 })=\frac { 75 }{ 100 } =\frac { 3 }{ 4 } ,\)
By Bayes' theorem
\(P({ E }_{ 1 }/A)=\frac { P({ E }_{ 1 })P(A/{ E }_{ 1 }) }{ P({ E }_{ 1 })P(A/{ E }_{ 1 })+P({ E }_{ 2 })P(A/{ E }_{ 2 })+P({ E }_{ 3 })P(A/{ E }_{ 3 }) } \)
\(=\frac { \frac { 1 }{ 3 } \times 1 }{ \frac { 1 }{ 3 } \times 1+\frac { 1 }{ 3 } \times \frac { 3 }{ 4 } +\frac { 1 }{ 3 } \times \frac { 3 }{ 5 } } \)
\(=\frac{4}{4+3+2}=\frac{4}{9}\)
8.
Let the events E1 and E2 be the events when lost card is a diamond and not a diamond respectively.
\(P({ E }_{ 1 })=\frac { 13 }{ 52 } =\frac { 1 }{ 4 } \)and
\(P({ E }_{ 2 })=\frac { 39 }{ 52 } =\frac { 3 }{ 4 } \)
Let A be the event:
"two cards drawn from the remaining pack are diamonds"
\(P(A/{ E }_{ 1 })=\frac { 12\times 11 }{ 51\times 50 } \)
\(P(A/{ E }_{ 2 })=\frac { 13\times 12 }{ 51\times 50 } \)
By Bayes' Theorem
\(P({ E }_{ 1 }/A)=\frac { P({ E }_{ 1 })P(A/{ E }_{ 1 }) }{ P({ E }_{ 1 })P(A/{ E }_{ 1 })+P({ E }_{ 2 })P(A/{ E }_{ 2 }) } \)
\(=\frac { \left( \frac { 1 }{ 4 } \right) \left( \frac { 12\times 11 }{ 51\times 50 } \right) }{ \left( \frac { 1 }{ 4 } \right) \left( \frac { 12\times 11 }{ 51\times 50 } \right) +\left( \frac { 3 }{ 4 } \right) \left( \frac { 13\times 12 }{ 51\times 50 } \right) } \)
\(=\frac { 12\times 11 }{ 12\times 11+3\times 13\times 12 } \)
\(=\frac { 132 }{ 132+468 } =\frac { 132 }{ 600 } =\frac { 11 }{ 50 } \)
9.
Let 'p' be the probability of success and 'q' be tha probability of failure.
Then p + q = 1 and p = 2q
Solving, \(p=\frac { 2 }{ 3 } ,q=\frac { 1 }{ 3 } \) Also n = 6.
Required probability = \(P(X\ge 4)\)
= P(4)+P(5)+P(6)
= \(^6C_4\)q2p4+\(^6C_5\)q1p5+\(^6C_6\)q0p6
= \(15{ \left( \frac { 1 }{ 3 } \right) }^{ 2 }{ \left( \frac { 2 }{ 3 } \right) }^{ 4 }+6{ \left( \frac { 1 }{ 3 } \right) }{ \left( \frac { 2 }{ 3 } \right) }^{ 5 }+\left( 1 \right) \left( 1 \right) { \left( \frac { 2 }{ 3 } \right) }^{ 6 }\)
= \({ \left( \frac { 2 }{ 3 } \right) }^{ 4 }\left[ \frac { 15 }{ 9 } +\frac { 4 }{ 3 } +\frac { 4 }{ 9 } \right] =\frac { 31 }{ 9 } { \left( \frac { 2 }{ 3 } \right) }^{ 4 }\)
10.
Let the events be:
E1 : Selected person is a male.
E2 : Selected person is afemale
And A : Selected person is grey haired.
\(P({ E }_{ 1 })=P({ E }_{ 2 })=\frac { 1 }{ 2 } \)
\(P(A/{ E }_{ 1 })=\frac { 5 }{ 100 } =\frac { 1 }{ 20 } \)
and \(P(A/{ E }_{ 2 })=\frac { 0.25 }{ 100 } =\frac { 1 }{ 400 } \)
Required probability
= \(\frac { P({ E }_{ 1 }).P(A/{ E }_{ 1 }) }{ P({ E }_{ 1 })P(A/{ E }_{ 1 })+P({ E }_{ 2 })P(A/{ E }_{ 2 }) } \)
= \(\frac { \left( \frac { 1 }{ 2 } \right) \left( \frac { 1 }{ 20 } \right) }{ \left( \frac { 1 }{ 2 } \right) \left( \frac { 1 }{ 20 } \right) +\left( \frac { 1 }{ 2 } \right) \left( \frac { 1 }{ 400 } \right) } =\frac { \frac { 1 }{ 20 } }{ \frac { 1 }{ 20 } +\frac { 1 }{ 400 } } =\frac { 20 }{ 21 } \)
11.
Let the events be:
E1 : Student resides in the hostel
E2 : Student is a day scholar
and A : Student attains 'A' grade.
\(P({ E }_{ 1 })=\frac { 60 }{ 100 } =\frac { 3 }{ 5 } \)
\(P({ E }_{ 2 })=\frac { 40 }{ 100 } =\frac { 2 }{ 5 } \)
and \(P(A/{ E }_{ 1 })=\frac { 30 }{ 100 } =\frac { 3 }{ 10 } \)
\(P(A/{ E }_{ 2 })=\frac { 20 }{ 100 } =\frac { 2 }{ 10 } \)
By Bayes' Theorem,
\(P({ E }_{ 1 }/A)=\frac { P({ E }_{ 1 })P(A/{ E }_{ 1 }) }{ P({ E }_{ 1 })P(A/{ E }_{ 1 })+P({ E }_{ 2 })P(A/{ E }_{ 2 }) } \)
\(=\frac { \left( \frac { 3 }{ 5 } \right) \left( \frac { 3 }{ 10 } \right) }{ \left( \frac { 3 }{ 5 } \right) \left( \frac { 3 }{ 10 } \right) +\left( \frac { 2 }{ 5 } \right) \left( \frac { 2 }{ 10 } \right) } =\frac { 9 }{ 9+4 } =\frac { 9 }{ 13 } \)
12.
Let the events be as below:
E1 : 2 red balls are transfered from Bag I to Bag II
E2 : 2 black balls are transferd frm Bag I to Bag II
E3 : 1 red and 1 black balls are transferd from Bag I to Bag
and A : 1 red ball is drawn from Bag II
\(P({ E }_{ 1 })=\frac { ^{ 3 }{ C }_{ 2 } }{ ^{ 7 }{ C }_{ 2 } } =\frac { 3\times 2 }{ 7\times 6 } =\frac { 1 }{ 7 } \)
\(P({ E }_{ 1 })=\frac { ^{ 4 }{ C }_{ 2 } }{ ^{ 7 }{ C }_{ 2 } } =\frac { 4\times 3 }{ 7\times 6 } =\frac { 2 }{ 7 } \)
\(P({ E }_{ 3 })=\frac { ^{ 3 }{ C }_{ 1 }\times ^{ 4 }{ C }_{ 1 } }{ ^{ 7 }{ C }_{ 2 } } =\frac { 3\times 4 }{ \frac { 7\times 6 }{ 1\times 2 } } =\frac { 3\times 4 }{ 7\times 3 } =\frac { 4 }{ 7 } \)
and \(P(A/{ E }_{ 1 })=\frac { 6 }{ 11 } ,P(A/{ E }_{ 2 })=\frac { 4 }{ 11 } ,P(A/{ E }_{ 3 })=\frac { 5 }{ 11 } \)
By Bayes' Theorem
\(P({ E }_{ 2 }/A)=\frac { P({ E }_{ 2 })P(A/{ E }_{ 2 }) }{ P({ E }_{ 1 })P(A/{ E }_{ 1 })+P({ E }_{ 2 })P(A/{ E }_{ 2 })+P({ E }_{ 3 })P(A/{ E }_{ 3 }) } \)
\(=\frac { \left( \frac { 2 }{ 7 } \right) \left( \frac { 4 }{ 11 } \right) }{ \left( \frac { 1 }{ 7 } \right) \left( \frac { 6 }{ 11 } \right) +\left( \frac { 2 }{ 7 } \right) \left( \frac { 4 }{ 11 } \right) +\left( \frac { 4 }{ 7 } \right) \left( \frac { 5 }{ 11 } \right) } \)
\(=\frac { 8 }{ 6+8+20 } =\frac { 8 }{ 34 } =\frac { 4 }{ 17 } \)
13.
Let the events be:
E1 : The girl gets 1, 2, 3 and 4 on the dice
E2 : The girl gets 5 or 6 on the dice.
A : Exactly one head shows up.
\(P({ E }_{ 1 })=\frac { 4 }{ 6 } =\frac { 2 }{ 3 } ,P({ E }_{ 2 })=\frac { 2 }{ 6 } =\frac { 1 }{ 3 } \)
and \(P(A/{ E }_{ 1 })=\frac { 1 }{ 2 } ,P(A/{ E }_{ 2 })=\frac { 3 }{ 8 } \)
By Bayes' Theorem,
\(P({ E }_{ 1 }/A)=\frac { P({ E }_{ 1 })P(A/{ E }_{ 1 }) }{ P({ E }_{ 1 })P(A/{ E }_{ 1 })+P({ E }_{ 2 })P(A/{ E }_{ 2 }) } \)
\(=\frac { \frac { 2 }{ 3 } \times \frac { 1 }{ 2 } }{ \frac { 2 }{ 3 } \times \frac { 1 }{ 2 } +\frac { 1 }{ 3 } \times \frac { 3 }{ 8 } } =\frac { \frac { 1 }{ 3 } }{ \frac { 1 }{ 3 } +\frac { 1 }{ 8 } } =\frac { 8 }{ 11 } \)
14.
Let A be the event ‘the number on the card drawn is even and B be the event ‘the number on the card drawn is greater than 3. We have to find P(A|B).
Now, the sample space of the experiment is S = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
Then A = {2, 4, 6, 8, 10}, B = {4, 5, 6, 7, 8, 9, 10}
and A ∩ B = {4, 6, 8, 10}
\(\text { Also } \quad \mathrm{P}(\mathrm{A})=\frac{5}{10}, \mathrm{P}(\mathrm{B})=\frac{7}{10} \text { and } \mathrm{P}(\mathrm{A} \cap \mathrm{B})=\frac{4}{10}\)
Then \(P(A|B)=\frac { P(A\cap B) }{ P(B) } =\frac { \frac { 4 }{ 10 } }{ \frac { 7 }{ 10 } } =\frac { 4 }{ 7 } \)
15.
\(\text {(i) } E=\{1,3,5\}, F=\{2,3\}, E \cap F=\{3\}\)
\(P(E)=\frac{3}{6}, P(F)=\frac{2}{6}, P(E \cap F)=\frac{1}{6},\)
\(\mathrm{P}(\mathrm{E} / \mathrm{F})=\frac{\mathrm{P}(\mathrm{E} \cap \mathrm{F})}{\mathrm{P}(\mathrm{F})}=\frac{1}{6} \div \frac{2}{6}=\frac{1}{2} \)
\(\mathrm{P}(\mathrm{FE})=\frac{\mathrm{P}(\mathrm{E} \cap \mathrm{F})}{\mathrm{P}(\mathrm{E})}=\frac{1}{6} \div \frac{3}{6}=\frac{1}{3} \)
\(\text {(ii) } \mathrm{E}=\{1,3,5\}, \mathrm{G}=\{2,3,4,5\}, \mathrm{E} \cap \mathrm{G}=\{3,5\} \)
\(\mathrm{P}(\mathrm{E})=\frac{3}{6}, \mathrm{P}(\mathrm{G})=\frac{4}{6}, \mathrm{P}(\mathrm{E} \cap \mathrm{G})=\frac{2}{6} \)
\(\mathrm{P}(\mathrm{E} / \mathrm{G})=\frac{\mathrm{P}(\mathrm{E} \cap \mathrm{G})}{\mathrm{P}(\mathrm{G})}=\frac{2}{6} \div \frac{4}{6}=\frac{2}{4}=\frac{1}{2} \)
\(\mathrm{P}(\mathrm{G} / \mathrm{E})=\frac{\mathrm{P}(\mathrm{E} \cap \mathrm{G})}{\mathrm{P}(\mathrm{E})}=\frac{2}{6} \div \frac{3}{6}=\frac{2}{3} \)
\(\text {(iii) } \mathrm{E}=\{1,3,5\}, \mathrm{F}=\{2,3\}\}, \mathrm{G}=\{2,3,4,5\} \)
\(\mathrm{E} \cap \mathrm{G}=\{3,5\}, \mathrm{F} \cap \mathrm{G}=\{2,3\}, \)
\((\mathrm{E} \cap \mathrm{F}) \cap \mathrm{G}=\{3\} \)
\(\mathrm{P}(\mathrm{E} \cap \mathrm{G})=\frac{2}{6}, \mathrm{P}(\mathrm{F} \cap \mathrm{G})=\frac{2}{6}, \mathrm{P}[(\mathrm{E} \cap \mathrm{F}) \cap \mathrm{G}]=\frac{1}{6} \)
\( \mathrm{Now} \mathrm{P}(\mathrm{E} \cup \mathrm{F} / \mathrm{G}) \)
\(=\mathrm{P}(\mathrm{E} / \mathrm{G})+\mathrm{P}(\mathrm{F} / \mathrm{G})-\mathrm{P}[(\mathrm{E} \cap \mathrm{F}) / \mathrm{G}] \)
\(=\frac{\mathrm{P}(\mathrm{E} \cap \mathrm{G})}{\mathrm{P}(\mathrm{G})}+\frac{\mathrm{P}(\mathrm{E} \cap \mathrm{G})}{\mathrm{P}(\mathrm{G})}-\frac{\mathrm{P}[\mathrm{E} \cap \mathrm{F}) \cap \mathrm{G}]}{\mathrm{P}(\mathrm{G})} \)
\(=\left(\frac{2}{6} \div \frac{4}{6}\right)+\left(\frac{2}{6} \div \frac{4}{6}\right)-\left(\frac{1}{6} \div \frac{4}{6}\right) \)
\(=\frac{2}{4}+\frac{2}{4}-\frac{1}{4}=\frac{3}{4} \)
\(\mathrm{P}(\mathrm{E} \cap \mathrm{F} / \mathrm{G})=\frac{\mathrm{P}[\mathrm{E} \cap \mathrm{F}) \cap \mathrm{G}]}{\mathrm{P}(\mathrm{G})}=\frac{1}{6} \div \frac{4}{6}=\frac{1}{6} \)
16.
\(P(\overset { \_ }{ A } \cup \overset { \_ }{ B } )=1-P(A\cap B)\)
\(\frac { 1 }{ 4 } =1-P(A\cap B)\)
\(P(A\cap B)=1-\frac { 1 }{ 4 } =\frac { 3 }{ 4 } \)
And P(A)P(B) = \(\frac { 1 }{ 2 } \times \frac { 7 }{ 12 } =\frac { 7 }{ 24 } \)
Thus \(P(A\cap B)\neq P(A)P(B)\)
Hence, A and B are not independent.
17.
Here \({ \left( \frac { 1 }{ 2 } +\frac { 1 }{ 2 } \right) }^{ 6 }\)
= \(^{ 6 }{ C }_{ 0 }{ \left( \frac { 1 }{ 2 } \right) }^{ 6 }+^{ 6 }{ C }_{ 1 }{ \left( \frac { 1 }{ 2 } \right) }^{ 6 }+...........+^{ 6 }{ C }_{ 6 }{ \left( \frac { 1 }{ 2 } \right) }^{ 6 }\)
= \({ \left( \frac { 1 }{ 2 } \right) }^{ 6 }\left[ ^{ 6 }{ C }_{ 0 }+^{ 6 }{ C }_{ 1 }+........+^{ 6 }{ C }_{ 6 } \right] \)
= \({ \left( \frac { 1 }{ 2 } \right) }^{ 6 }\left[ ^{ 6 }{ C }_{ 0 }+^{ 6 }{ C }_{ 1 }+^{ 6 }{ C }_{ 2 }+^{ 6 }{ C }_{ 3 }+^{ 6 }{ C }_{ 2 }+^{ 6 }{ C }_{ 1 }+^{ 6 }{ C }_{ 0 } \right] \)
\(^6C_3 \)has the max.value in \(^{ 6 }{ C }_{ 0 },^{ 6 }{ C }_{ 1 },^{ 6 }{ C }_{ 2 }\)and \(^{ 6 }{ C }_{ 3 }\)
Here \(^{ 6 }{ C }_{ 3 }{ \left( \frac { 1 }{ 2 } \right) }^{ 6 }\) is maximum.
Hence, P(X = 3) is most likely outcome.
18.
E : Son on one end = {(s, m, f), (s, f, m), (f, m, s), (m, f, s)}
No.of exhaustive cases = 3! = 6
where s = son, m = mother and f = father
F : Father in middle = {(m, s ,f), (s, f, m)},
\(E\cap F\)={(m, s, f), (s, f, m)}
\(P(E\cap F)=\frac { 2 }{ 6 } =\frac { 1 }{ 3 } \)
and \(P(F)=\frac { 2 }{ 6 } =\frac { 1 }{ 3 } \)
\(P(E/F)=\frac { P(E\cap F) }{ P(F) } =\frac { \frac { 1 }{ 3 } }{ \frac { 1 }{ 3 } } =1\)
19.
We have:
P(A) = 0.3 and P(B) = 0.4
(i) Since A and B are independent,
\(P(A\cap B)=P(A)P(B)\)
= (0.3) (0.4) = 0.12
(ii) \(P(A\cup B)=P(A)+P(B)-P(A\cap B)\)
= 0.3 + 0.4 - 0.12 = 0.7 - 0.12 = 0.58
(iii) \(P(A/B)=\frac { P(A\cap B) }{ P(B) } =\frac { 0.12 }{ 0.4 } \)
= \(\frac { 12 }{ 100 } \times \frac { 10 }{ 4 } \)
= \(\frac { 12 }{ 10 } =\frac { 3 }{ 10 } \)
(iv) \(P(B/A)=\frac { P(A\cap B) }{ P(A) } =\frac { 0.12 }{ 0.4 } =\frac { 12 }{ 100 } \times \frac { 10 }{ 3 } =\frac { 12 }{ 10 } =\frac { 2 }{ 5 } \)
20.
The sample space has 216 outcomes.
\(\text { Now } \quad \mathrm{A}=\left\{\begin{array}{lllll} (1,1,4) & (1,2,4) & \ldots & (1,6,4) & (2,1,4) & (2,2,4) & \ldots (2,6,4) \\ (3,1,4) & (3,2,4) & \ldots &(3,6,4) & (4,1,4) & (4,2,4) & \ldots(4,6,4) \\ (5,1,4) & (5,2,4) & \ldots & (5,6,4) & (6,1,4) & (6,2,4) & \ldots(6,6,4) \end{array}\right\}\)
B = {(6,5,1), (6,5,2), (6,5,3), (6,5,4), (6,5,5), (6,5,6)} and A ∩ B = {(6,5,4)}.
\(\text { Now }P(B)=\frac{6}{216} \text { and } P(A \cap B)=\frac{1}{216} \)
Then \(P(A|B)=\frac { P(A\cap B) }{ P(B) } =\frac { \frac { 1 }{ 216 } }{ \frac { 6 }{ 216 } } =\frac { 1 }{ 6 }\)
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