The average age of a family(two parents and their children) is 20. If we exclude one parent, who is 40, the average age drops to 15. How many children are there in the family? Thanks for help.

Let the number of children =C

Let the sum total of the whole family's ages =S

S/(C+2)=20..........................(1)

(S-40) /(C+1) =15.................(2), solve for C, S

C = 3 children in the family

S =100 - The family combined ages.

Let the sum of all ages = A

And let N be the number of family members.......then we have two equations

A = 20N

A - 40 = 15(N - 1) → A = 15(N-1) + 40

Setting these equal, we have

20N = 15(N-1) + 40

20N = 15N + 25

5N = 25

N = 5

Thus.....we have 2 adults and 3 children