A father is 30 years older than his son. In 5 years, the father will be three times as old as the son. What is the son's present age?

Explanation

1. 01

   Let the son be $s$; the father is $s + 30$.

2. 02

   In 5 years: $s + 35 = 3(s + 5)$.

3. 03

   $s + 35 = 3s + 15$, so $2s = 20$ and $s = 10$.

4. 04

   Check: in 5 years they are 15 and 45, and $45 = 3 \times 15$.