The mean of the ages of ten secondary school pupils is 16, but when the age of their teacher is added, the mean becomes 19. Find the age of the teacher.
Worked solution (try it first)
Total age of the 10 pupils: 10×16=160 years.
With the teacher there are 11 people: 11×19=209 years.
The teacher's age is the difference: 209−160=49, option D.