we have 100 families: 10 families have no children, 40 families have 1 child for each one, 30 families have 2 children for each one, 10 families have 3 children for each one and 10 families have 4 children for each one
A- suppose we selected a family randomly, what is the expected number of children in that family? B-suppose we selected a child randomly, what is the expected number of children in his family? is it the same?
I've tried the expectation to solve it but only for "A" , for "B" I didn't know how to solve it
the answer for A is 1.7 children but it is wrong because prof said the correct one is 3.