In a family of 21 people, the average age is 14 years. If the age of the grandfather is not counted, the average age drops to 12 years. What is the age of the grandfather?
Worked solution (try it first)
Total of all 21 ages: 21×14=294 years.
Total of the other 20 ages: 20×12=240 years.
The grandfather is the difference: 294−240=54 years, option D.