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Published on: 04/12/2019
Probability
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1.
A die is tossed thrice. Find the probability of getting an odd number at least once.
2.
If P(A) = \(\frac { 3 }{ 5 } \) and P(B) = \(\frac { 1 }{ 5 } \) find \(P(A\cap B)\) if A and B are independent events.
3.
Given that E and F are events such that:
P(E) = 0.6, P(F) = 0.3 and \(P(E\cap F)\) = 0.2 find P(E|F) and P(F|E).
4.
A die is thrown. If E is the event ‘the number appearing is a multiple of 3’ and F be the event ‘the number appearing is even’ then find whether E and F are independent ?
5.
Three cards are drawn successively, without replacement from a pack of 52 well shuffled cards. What is the probability that first two cards are kings and the third card drawn is an ace?
6.
12 cards, numbered 1 to 12, are placed in box mixed up thoroughly and then a card is drawn at random from the box. If it is known that the number on the drawn card is more than 3, find the probability that it is an even number.
7.
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.
8.
If each element of a second order determinant is either 0 or 1, what is the probability that the value of determinant is positive? (Assume that the individual entries of the determinant are chosen independently, each value assumed with probability \(1\over2\)).
9.
If P(A)=\(\frac { 2 }{ 5 } ,P(B)=\frac { 1 }{ 3 } ,P(A\cap B)=\frac { 1 }{ 5 } ,\) then find \(P(\bar { A } /\bar { B } )\) .
10.
If P(F) = 0.35 and P(E\(\cup\)F) = 0.85 and E and F are independent events. Find P(E).
11.
If E and F are two events such that \(P(E)=\frac { 1 }{ 4 } ,\) \(P(E)=\frac { 1 }{ 2 } \) and \( P(E\cap F)=\frac { 1 }{ 8 } \), find
(a) P(E or F)
(b) P(not E and not F).
12.
A bag contain 2 red, 6 black and 8 green balls. A ball is drawn at random from the bag. Find the probabilty:
(a) a red ball
(b) a black ball
(c) a green ball
(d) a non-red ball
13.
Consider the experiment of throwing a die, if a multiple of 3 comes up, throw the die again and if any other number comes, toss a coin. Find the conditional probability of the event ‘the coin shows a tail’, given that ‘at least one die shows a 3’.
14.
Events E and F are given to be independent. Find P(F) if it is given that P(E) = 0.60 and P(E\(\cap\)F) = 0.35
15.
Given P(A) = 0.2, P(B) = 0.3 and \(P(A\cap B)=0.3\) Find P(A/B)
16.
Bayes’ Theorem If E1 , E2 ,..., En are n non empty events which constitute a partition of sample space S, i.e. E1 , E2 ,..., En are pairwise disjoint and E1∪ E2∪ ... ∪ En = S and A is any event of nonzero probability, then
\(\mathrm{P}\left(\mathrm{E}_i \mid \mathrm{A}\right)=\frac{\mathrm{P}\left(\mathrm{E}_i\right) \mathrm{P}\left(\mathrm{A}_{\mid} \mathrm{E}_i\right)}{\sum_{j=1}^n \mathrm{P}\left(\mathrm{E}_j\right) \mathrm{P}\left({\left.\mathrm{A} \mid E_j\right)}_1\right.} \text { for any } i=1,2,3, \ldots, n\)
17.
Given P(A) = \(1\over2\), P(B) = \(1\over3\) and \(P(A\cap B)={1\over6}\) Are the events A and B independent?
18.
Two numbers are selected at random (without replacement) from the first six positive integers.Let x denote the larger of the two numbers obtained.Find the probability distribution of random variable x and hence find the mean of the distribution.
19.
An urn contains 4 balls. Two balls are drawn at random from the urn (without replacement) and are found to be white, What is the probability that all the four balls in the urn are white?
1.
P(getting on odd number) = \(\frac { 3 }{ 6 } \)
= \(\frac { 1 }{ 2 } \) [\(\because \) 1, 3, 5 are odd]
\(\therefore \) P(getting an even number) = \(1-\frac { 1 }{ 2 } =\frac { 1 }{ 2 } \)
\(\therefore \) Probability of getting an odd number on throw of adie 3 times = Probability of getting an even number in a throw of a die 3 times
= \(\frac { 1 }{ 2 } \times \frac { 1 }{ 2 } \times \frac { 1 }{ 2 } =\frac { 1 }{ 8 } \)
Hence,probability of at least one odd number in athrow of adie 3 times
= \(1-\frac { 1 }{ 8 } =\frac { 7 }{ 8 } \)
2.
Since A and B are independent events
\(\therefore \) \(P(A\cap B)=P(A)P(B)\)
\(=\left( \frac { 3 }{ 5 } \right) \left( \frac { 1 }{ 5 } \right) =\frac { 3 }{ 25 } \)
3.
(i) \(P(E/F)=\frac { P(E\cap F) }{ P(F) } =\frac { 0.2 }{ 0.3 } =\frac { 2 }{ 3 } \)
(ii) \(P(F/E)=\frac { P(E\cap F) }{ P(E) } =\frac { 0.2 }{ 0.6 } =\frac { 1 }{ 3 }\)
4.
We know that the sample space is S = {1, 2, 3, 4, 5, 6}
Now E = { 3, 6}, F = { 2, 4, 6} and E ∩ F = {6}
Then \(\mathrm{P}(\mathrm{E})=\frac{2}{6}=\frac{1}{3}, \mathrm{P}(\mathrm{F})=\frac{3}{6}=\frac{1}{2} \text { and } \mathrm{P}(\mathrm{E} \cap \mathrm{F})=\frac{1}{6}\)
Clearly P(E ∩ F) = P(E). P (F)
Hence E and F are independent events.
5.
Let K denote the event that the card drawn is king and A be the event that the card drawn is an ace.
Clearly, we have to find P (KKA)
Now P(K) = \(\frac { 4 }{ 52 } \)
Also, P (K|K) is the probability of second king with the condition that one king has already been drawn. Now there are three kings in (52 − 1) = 51 cards.
Therefore P(K/K) = \(\frac { 3 }{ 51 } \)
Lastly, P(A|KK) is the probability of third drawn card to be an ace, with the condition that two kings have already been drawn. Now there are four aces in left 50 cards.
Therefore P(A|KK) =\(\frac { 4 }{ 50 } \)
By multiplication law of probability, we have
Now P(KKA) = P(K) P(K/K) P(A/KK)
= \(\frac { 4 }{ 52 } \times \frac { 3 }{ 51 } \times \frac { 4 }{ 50 } =\frac { 2 }{ 5525 } \)
6.
Total cards are 12
A : number drawn is more than 3, i.e. 4, 5, 6, ..., 12.
B : getting an even number, i.e. 2, 4, 6, 8, 10, 12.
\(A \cap B: 4,6,8,10,12 \)
\(P(B / A)=\frac{P(A \cap B)}{P(A)}=\frac{5 / 12}{9 / 12}=\frac{5}{9} .
\)
7.
| X | 0 | 1 | 2 | 3 | 4 |
| P(x) | 625/1296 | 500/1296 | 150/1296 | 20/1296 | 1/1296 |
8.
There are four entries determinant of 2 x 2 order. Each entry may be filled up in two ways with 0 or 1. Therefore, number of determinants that can be formed
= 24 = 16
The value of determinant is positive in the following cases
\(\begin{vmatrix} 1 &0 \\0 &1 \end{vmatrix},\begin{vmatrix}1 &0 \\1 &1 \end{vmatrix},\begin{vmatrix}1 & 1 \\ 0 & 1 \end{vmatrix}\)
i.e, 3 determinants
Thus, the probability that the determinants is positive \(={3\over 16}\)
9.
\(P(\bar { A } /\bar { B } )=\frac { P(\bar { A } \cap \bar { B } ) }{ P(\bar { B } ) } \)
\(=\frac { 1-P(A\cup B) }{ 1-P(B) } \)
\(=\frac { 1-[P(A)+P(B)-P(A\cap B)] }{ 1-P(B) } \)
\(=\frac { 7 }{ 10 } \)
10.
\(P(E\cap F)=P(E)\times P(F)\)
\(P(E\cup F)=P(E)+P(F)-P(E\cap F)\)
\(\Rightarrow P(E\cup F)=P(E)+P(F)-P(E)\times P(F)\)
\(\Rightarrow 0.85=P(E)+0.35-P(E)\times 0.35\)
\(\Rightarrow 0.85=P(E)(1-0.35)+0.35\)
\(\Rightarrow 0.85-0.35=P(E)(0.65)\)
\(\Rightarrow \frac { 0.52 }{ 0.65 } =P(E)\)
\(\Rightarrow P(E)=\frac { 50 }{ 65 } =\frac { 10 }{ 13 } \)
\(\therefore P(F)=1-\frac { 10 }{ 13 } =\frac { 3 }{ 13 } \)
11.
Given, \(P(E)=\frac { 1 }{ 4 } \)
\(P(F)=\frac { 1 }{ 2 } \)
and \(P(E\cap F) =\frac { 1 }{ 8 } \)
(a) P(E or F) = (E\(\cup\)F) = P(E) + P(F) - P(E\(\cap\)F)
\(=\frac { 1 }{ 4 } +\frac { 1 }{ 2 } -\frac { 1 }{ 8 } \)
\(=\frac { 2+4-1 }{ 8 } \)
\(=\frac { 5 }{ 8 } \)
(b) P(not E and not F) = \(P(\bar { E } \cap \bar { F } )=P\overline { (E\cup F) } \)
= 1 - P(E\(\cup\) F)
= 1 - \(\frac { 5 }{ 8 } =\frac { 3 }{ 8 } \)
12.
Total number of cards = 2 + 6 + 8 = 16
(a) Number of red balls = 2
\(\therefore\) Required probability = \(\frac { 2 }{ 16 } =\frac { 1 }{ 8 } \)
(b) Number of black balls = 6
\(\therefore\) Required probability = \(\frac { 6 }{ 16 } =\frac { 3 }{ 8 } \)
(c) number of green balls = 8
\(\therefore\) Required probability = \(\frac { 8 }{ 16 } =\frac { 1 }{ 2 } \)
(d) Number of non-red balls = 14
\(\therefore\) Required probability = \(\frac { 14 }{ 16 } \)
13.
The outcomes of the given experiment can be represented by the following tree diagram.
The sample space of the experiment is,
Let be the event that the coin shows a tail and be the event that at least one die shows .
Then,
Probability of the event that the coin shows a tail, given that at least one die shows , is given by .
Therefore, \(P(A \mid B)=\frac{P(A \cap B)}{P(B)}=\frac{0}{\frac{7}{36}}=0\)
14.
For independent events,
P(E∩F) = P(E) ⋅ P(F)
\(\Rightarrow 0.35=0.60 \times P(F) \Rightarrow P(F)=\frac{7}{12}=0.58\)
15.
1/3
16.
proof :
By formula of conditional probability, we know that
\( \mathrm{P}\left(\mathrm{E}_i \mid \mathrm{A}\right) =\frac{\mathrm{P}\left(\mathrm{A} \cap \mathrm{E}_i\right)}{\mathrm{P}(\mathrm{A})} \)
\(=\frac{\mathrm{P}\left(\mathrm{E}_i\right) \mathrm{P}\left(\mathrm{AlE} \mathrm{E}_i\right)}{\mathrm{P}(\mathrm{A})}(b y \) (by multiplication rule of probability)
\( =\frac{\mathrm{P}\left(\mathrm{E}_i\right) \mathrm{P}\left(\mathrm{AlE}_i\right)}{\sum_{j=1}^n \mathrm{P}\left(\mathrm{E}_j\right) \mathrm{P}\left(\mathrm{AlE}_j\right)} \)(by the result of theorem of total probability)
17.
P(A) ⋅ P(B) = \(1\over2\)⋅\(1\over3\) = \(1\over6\) = P(A∩B)
Yes, the events are independent.
18.
Total number of ways of selecting two numbers = 6C2 = 15
Values of x (larger of the two) can be 2, 3, 4, 5, 6
\(P(x=2)={1\over 15}\)
\(P(x=3)={2\over 15}\)
\(P(x=4)={3\over 15}\)
\(P(x=5)={4\over 15}\)
and \(P(x=6)={5\over 15}\)
Distribution can be written as
| x | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|
| P(x) | \(1\over 15\) | \(2\over 15\) | \(3\over 15\) | \(4\over 15\) | \(5\over 15\) |
| xP(x) | \(2\over 15\) | \(6\over 15\) | \(12\over 15\) | \(20\over 15\) | \(30\over 15\) |
Mean \(=\sum xP(x)={70\over 15}={14\over 3}\)
19.
Let E1: urn has 2 white balls
E2: urn has 3 white balls
E3: urn has 4 white balls
A: 2 balls drawn are white
P(E1) = P(E2) = P(E3) = \(\frac { 1 }{ 3 } \)
P(A/E1)=\(\frac { { 2 }_{ { C }_{ 2 } } }{ { 4 }_{ { C }_{ 2 } } } =\frac { 1 }{ 6, } \)
\(P(A/{ E }_{ 2 })=\frac { { 2 }_{ { C }_{ 2 } } }{ { 4 }_{ C_{ 2 } } } =\frac { 1 }{ 6 } ,\)
\(P(A/{ E }_{ 2 })=\frac { { 3 }_{ { C }_{ 2 } } }{ { 4 }_{ C_{ 2 } } } =\frac { 1 }{ 2 } ,\)
\(P(A/{ E }_{ 3 })=\frac { { 4 }_{ { C }_{ 2 } } }{ { 4 }_{ { C }_{ 2 } } } =1\)
\(\therefore P({ E }_{ 3 }/A)\)
\(=\frac { { P(E }_{ 3 }).P(A/{ E }_{ 3 }) }{ P({ E }_{ 1 })P(A/{ E }_{ 1 })+P({ E }_{ 2 })P(A/{ E }_{ 2 })+P({ E }_{ 3 })P(A/{ E }_{ 3 }) } \)
\(=\frac { \frac { 1 }{ 3 } .1 }{ \frac { 1 }{ 3 } .\frac { 1 }{ 6 } +\frac { 1 }{ 3 } .\frac { 1 }{ 2 } +\frac { 1 }{ 3 } .1 } \)
\(=\frac { 6 }{ 10 } =0.6\)
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