**Four years ago, the average age of a family of four persons was 18 years. During this period, a baby was born. Today if the average age of the family is still 18 years, the age of the baby is?**

### Note

The sum of the ages of 4 members, 4 years ago = (18 × 4) years = 72 years.

The sum of the ages of 4 members now = (72 + 4 × 4) years = 88 years.

The sum of the ages of 5 members now = (18 × 5) years = 90 years.

**∴ Age of the baby = (90 - 88) years = 2 years.**