B was born when A was 4 years 7 months old, and C was born when B was 3 years 4 months old. When C was 5 years 2 months old, what was the average age of A, B and C?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 8 years 11 months
The age gaps stay the same all life, so the ages of B and A follow from the age of C. Working in months avoids carrying.
Age of C \(=5\) years 2 months \(=62\) months
Age of B \(=62+(3\times12+4)=62+40=102\) months
Age of A \(=102+(4\times12+7)=102+55=157\) months
\(\displaystyle \text{Average}=\frac{62+102+157}{3}=\frac{321}{3}=107\) months
107 months \(=8\times12+11\) months \(=8\) years 11 months
Their average age was 8 years 11 months.