probability question(need explanation)

[kat4ever]

me qestion ekata answer eka 5/26.i need to know how to get that answer.

A family of five children is known to have at least two girls. What is the probability of this family having exactly four girls?

 

[rclakmal]

I think you have done the theorem ( I dont remember the name ) which says when there are n different objects you can have n! permutations .And in that n objects if there are p objects of 1 type and q number of 2nd type.(where p+q=n) then you can have n!/(p!*q!) .

So here are the possibilities

1. 2 girls 3 boys
permutations =5!/(2!*3!)=10

2. 3 girls 2 boys
permutations =5!/(3!*2!)=10

3. 4 girls 1 boy
permutations =5!/(4!*1!)=5

4. 5 girls 0 boys
permutations =5!/(2!*3!)=1

So all permutations =10+10+5+1=26
Your answer=5/26

Ask if you need any further help !