A father is four times as old as his son. After 10 years the father’s age will be 2.5 times that of his son. Find the son‘s present age.

Correct option is C

Given:

A father is four times as old as his son.

After 10 years, the father’s age will be 2.5 times that of his son. 

Solution:

Let the son's present age be x years.

The father's present age is 4 times that of the son, so the father's age is 4x years. 

After 10 years:

The son's age will be $x+10$ years.

The father's age will be $4x+10$ years.

According to the problem, after 10 years, the father’s age will be 2.5 times the son's age. Therefore,

​$$4x+10=2.5(x+10)$$ 

$$4x+10=2.5x+25$$ 

$$4x−2.5x=25-10$$ 

$$1.5x=15$$ 

$$x= \frac{15}{1.5}=10$$ 

Thus the son's present age is 10 years.