Question

3 years ago the average age of a family 5 members was 17 years. A baby having been born , the average family is the same today. The present age of the baby today ?

Answer 1

the SUM of their ages was (5)(17) = 85 years.

A baby was born (meaning that there are now 6 family members). We're told that the PRESENT average age of the family members is the SAME.

This means that the sum of the ages is (6)(17) = 102 years.

We're asked for the PRESENT age of the baby.

Since each of the original 5 family members has aged 3 years since the initial average was calculated, the sum of their PRESENT ages is...

85 + (5)(3) = 100.

Since the current sum of the 6 family members is 102, the baby is 102-100 = 2 years old.