Correct Answer - Option 2 : 37

**Given:**

The average age of four sisters is 12 yr. If the age of their teacher be also included, the average is increased by 5 year.

**Formula:**

Average of n numbers a1, a2, .. …. an = (a1 + a2 + ... + an)/n

**Calculation:**

Total age of 4 sisters = 12 × 4 = 48

Since the average increases by 5 so new average becomes 17

So total age of all 5 including teacher = 17 × 5 = 85

The age of teacher = 85 - 48 = 37 yrs

**∴ The age of the teacher is 37 yrs.**