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.