Ten years ago, a father was three times as old as his son. Ten years from now, he will be twice as old as his son. What is the son’s present age?

Let the present age of the son = $x$ years
Then present age of the father = $y$ years

Condition 1

Ten years ago:
Father’s age = $y - 10$
Son’s age = $x - 10$

According to the question:
$y - 10 = 3(x - 10)$

$y - 10 = 3x - 30$
$y = 3x - 20$ …… (1)

Condition 2

Ten years from now:
Father’s age = $y + 10$
Son’s age = $x + 10$

Given:
$y + 10 = 2(x + 10)$

$y + 10 = 2x + 20$
$y = 2x + 10$ …… (2)

Solve (1) and (2)

$3x - 20 = 2x + 10$

$x = 30$