The average age of a husband and his wife was 23 year sat the time of their marriage. After 5 years, they have a one-year old child. The average age of the family now is

The key is to track the total of ages, not the average. At marriage, the husband and wife averaged 23, so their combined age was 46 years.

Five years later, each adult has aged 5 years, so the couple's combined age is 46 + 10 = 56 years. The one-year-old child adds 1 year, making the family's total 57 years across three members.

Average = 57 / 3 = 19 years, so A is correct.

• B (23 years) assumes the average never moves. It ignores that a newborn pulls the average down.
• C (28.5 years) is 57 / 2. It divides by two, which leaves the child out of the headcount even though the child is counted in the sum.
• D (29.3 years) has no valid calculation behind it. It is 88 / 3, which does not match the family's total age.

Core concept: an average is total divided by headcount, so any change in membership or ages has to be tracked through the total. This is a standard Prelims average-of-family problem, and it tests whether you hold the sum rather than the mean.

Trap: adding 5 years to the average (23 + 5 = 28) and then dividing by three. It ignores that the child is new and that the adults' ages only rise by 10 in total.