Question

Two digits are selected at random from the digits 1 through 9

Answer 1

If the sum is even, find the probability that both the numbers are odd.

Solution:

Let A=Getting two odd numbers 

B=Getting the sum as an even number.

Required probability:

$P(A|B)=\frac{P(A\cap B)}{P(B)}=\frac{C^2_5/C^2_9}{(C^2_4+C^2_4)/C^2_9}=\frac{C^2_5}{C^2_4+C^2_5}=10/16=5/8$