The mean age of 30 pupils in a class is 15.3 years. One boy leaves the class and one girl is enrolled, and the new mean age of the 30 pupils becomes 15.2 years. How much older is the boy than the girl?
Worked solution (try it first)
The old total age is 30×15.3=459 years.
The new total is 30×15.2=456 years.
The number of pupils is unchanged, so the drop of 459−456=3 years is the boy's age minus the girl's.