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# The mean age of a class of twenty students is 12. If their teacher's age is inclu...

### Question

The mean age of a class of twenty students is 12. If their teacher's age is included, the mean becomes 13. The age of the teacher is

A) 33

B) 39

C) 28

D) 25

### Explanation:

Total age of 20 students = $$12 \times 20 = 240$$
Age of teacher = $$x$$
$$\cfrac{x + 240}{21} = 13$$
$$x + 240 = 13 \times 21$$
$$x + 240 = 273$$
$$x = 273 - 240$$
$$x = 33$$
Teacher's age = 33 years

## Dicussion (1)

• Total age of 20 students = $$12 \times 20 = 240$$
Age of teacher = $$x$$
$$\cfrac{x + 240}{21} = 13$$
$$x + 240 = 13 \times 21$$
$$x + 240 = 273$$
$$x = 273 - 240$$
$$x = 33$$
Teacher's age = 33 years