The average age of 20 boys of a class is 12 years. Ten new boys are admitted and the average age increases by a year. The average age of the new boys is ________ years?
The Usual Method
If the average age of 20 boys is 12 years, total age of the 20 boys = 20 x 12 = 240 years.
After admission of 10 new boys, new average age is13 years.
Therefore total age of class = years.
Hence total age of new boys = 390 – 240 = 150 years.
Hence average age of new boys = years.
Estimated Time to arrive at the answer = 45 seconds.
After admission of 10 new students, the total number of students becomes 20 + 10 = 30. If 10 students have to increase the average age by 1 year of 30 students, total increase needed = 1 x 30 = 30 years. This extra 30 years is from 10 students only. Hence extra age per new student = years. Hence average age of the 10 new students = 12 + 3 = 15 years.
Estimated Time to arrive at the answer = 10 seconds.