The average age of 75 boys in a hall is 25 years and the average age of 20 boys of that hall is 12 years whereas the average age of 10 boys of that hall is 15 years. Thus, find the average age of remaining boys of the hall?

Let total boys = 75, average age = 25 years. Total age = 75 × 25 = 1875 years. Given: 20 boys average = 12 years, total age = 20 × 12 = 240 years. 10 boys average = 15 years, total age = 10 × 15 = 150 years. Remaining boys = 75 - 20 - 10 = 45 boys. Total age of remaining = 1875 - 240 - 150 = 1485 years. Average of remaining = 1485 ÷ 45 = 33 years.