Question

2.
There are 25 girls and 15 boys in class XI and 30 boys and 20 girls in class XII. If a student
chosen from a class, selected at random, happens to be a boy, find the probability that he has
been chosen from class XII.​

Answer 1

Given:

There are 25 girls and 15 boys in class XI and 30 boys and 20 girls in class XII.

To find:

If a student  chosen from a class, selected at random, happens to be a boy, find the probability that he has  been chosen from class XII.​

Solution:

Class XI has 25 girls + 15 boys =  40 children

class XII has 20 girls + 30 boys = 50 children

The probability that a student chosen is of class XII

= 50 / (40 + 50) = 50 / 90

Again, the probability that a student that too boy chosen is of class XII

= 30 / 50

Therefore, the probability of boy and also chosen from class 12 is  

= 50/90 × 30/50

= 30/90

= 1/3

= 33.3%