Question

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:

• A. 81
• B. 77
• C. 79
• D. 83

Answer 1

Answer:

C. 79

Average of 39 students = 39 years
$\text{Total age of students} = 39 \times 39 = 1521$

When the teacher is included:
Total people = 40
New average = 40 years
$\text{Total age (including teacher)} = 40 \times 40 = 1600$

Teacher’s age:

$\text{Teacher's age} = 1600 - 1521 = 79$

Final Answer:

Teacher’s age = 79 years