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