Average = (sum of elements)/(number of elements) Let the number of students initially in the nursery class be N. Given, The average age of a nursery class is 3 years - The total sum of ages of all students in the nursery class = 3N Given, 10 new students with an average age 2.5 years are included in the same nursery class Total sum of ages of 10 new students = 10 × 2.5 = 25 years The total sum of ages of all students including new students = 3N + 25 The average age of a class with new students = 17/6 ⇒ 17/6 = (3N + 25/(N + 10) ⇒ 17(N + 10) = 6(3N + 25) ⇒ 17N + 170 = 18N + 150 ⇒ N = 20 ∴ Number of students initially in the nursery class is 20 There will be no change in the average 3 years after half semester there is no special condition provided for it. So the average will be 3 before 10 new students join the class.