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

Step-by-Step Solution1. Calculate the total age of the 4 persons 4 years ago: Average age = 18 years Number of persons = 4Total sum = $18 \times 4 = \{72 \{ years}}$2. Calculate what their total age should be today (without the baby):Since 4 years have passed, each of the 4 people has aged by 4 years.Total increase = $4 \text{ people} \times 4 \text{ years} = 16 \text{ years}$.Expected total age today = $72 + 16 = \mathbf{88 \text{ years}}$.3. Calculate the actual total age of the family today (with the baby):The family now has 5 members (4 original + 1 baby).The average age is still 18 years.Actual total sum = $18 \times 5 = \mathbf{90 \text{ years}}$.4. Find the age of the baby:The difference between the actual total age (with the baby) and the expected total age of the original members gives us the baby's age.Age of the baby = $90 - 88 = \mathbf{2 \text{ years}}$.