Question

The average of the students of a class is 12 years. The average age of 15 of them is 10 years and the average age of the remaining is 13 years. How many students are there in the class?

Answer 1

Let, the total number of students be, 's'

Sum of their ages = (12×s)=12 s

The sum of the ages of 15 students= (15×10)

                                                          = 150 years

The number of remaining students = (s-15)

                                                          = s- 15

∴ The sum of the ages of the remaining students = ( s- 15 ) ×13

                                                                                 = 13 s - 195

By condition,

12 s = 150 + 13 s - 195

⇒ 12 s - 13 s = -45

⇒ -s = -45

⇒ s = 45

∴ The number of student in the class is 45.

I hope it is helpful.