Mahler 2015 Exercise 3.21

Quote:

The number of children per family follows a Binomial Distribution m=4 q=.5. For a child chosen at random, how many siblings (brothers and sisters) does he have on average?

I have been trying a different solution than the one given. I view this as a conditional expectation problem.



What is the expected number of children in a family (less one) given that there is at least one child in the family?



I come up with

Mahler 2015 Exercise 3.21