The average age of 39 students of a class is 39 years. If the age of the teacher is also included, the average age of the whole group becomes 40 years. The age (in years) of the teacher is:

Correct option is C

Given:

Number of students = 39

Average age of students = 39 years

Average age of whole group (students + teacher) = 40 years

Formula Used:

Average age =  $$\frac{Total \ age }{ Number \ of \ people}$$​​

Total age = Average age × Number of people

Age of teacher = Total age (with teacher) - Total age (students only)

Solution:

Total age of students = 39 × 39 = 1521

Total age of students and teacher = 40 × 40 = 1600

Age of teacher = 1600 - 1521 = 79 years