Probability 3 Question 4

4. Two integers are selected at random from the set 1,2$,$...,11. Given that the sum of selected numbers is even, the conditional probability that both the numbers are even is

(2019 Main, 11 Jan I)

(a) 25

(b) 12

(c) 710

(d) 35

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Solution:

  1. In 1,2,3,.,11 there are 5 even numbers and 6 odd numbers. The sum even is possible only when both are odd or both are even.

Let A be the event that denotes both numbers are even and B be the event that denotes sum of numbers is even. Then, n(A)=5C2 and n(B)=5C2+6C2

Required probability

P(A/B)=P(AB)P(B)=5C2/11C2(6C2+5C2)11C2=5C26C2+5C2=1015+10=25



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