The average of 20 students is 12 years, if the teacher's age is included, average increases by one. The age of the teacher is:

Answer: Option B

Average of 20 students = 12 years
Total age of 20 students
= 20 × 12
= 240 years
When teacher included average become 13 years
Now, total age 20 students and teacher
= 13 × 21 = 273 years
∴ Age of teacher
= 273 - 240
= 33 years