How Many Kids Are In The Family?

A family, made up of 2 parents with
children, has an average age of 20.
If you exclude 1 parent, who is 40,
the average age drops to 15. How many kids are
in the family?

Let S be the sum of the ages of all the members in the family &
Let C be the number of children.

Hence from the 1st ser of information we can write;
S/(C + 2) = 20
i.e. S = 20(C + 2)
Hence;
S = 20C + 40 —[1]

From the second set of information we can write
(S – 40)/ (C + 1) = 15
i.e. S – 40 = 15(C + 1)
S – 40 = 15C + 15
Therefore;
S = 15C + 55 —–[2]

Therefore, from [1] & [2] we get;
S = 20C + 40 = 15C + 55
20C + 40 = 15C + 55
5C = 15
Therefore;
C = 3

Answer: Number of children = 3