In a class, there are 40 boys and their average age is 16 years. One boy, aged 17 years, leaving the class and another joining, the average age becomes 15.875 years The age of the new boy is :

Sum of age of 40 boys
= 16 × 40 = 640
New age of 40 boys
= 15.875 × 40 = 635
Difference = 640 - 635 = 5 years
17 - x = 5
x = 17 - 5 = 12 years

Alternate :
Average is decreased it means the boy who joined the class is younger than boy who leave the class.
Let the age of boy who join = x
17 - x = difference in average
$$\frac{17 - x}{40}$$ = 0.125
17 - x = 5
x = 12