The present age of a father is twice the present age of his son. Five years ago, the father was three times as old as his son. What is the present age of the son?

Let the son's present age be x years. Then the father's present age is 2x years. Five years ago, son was (x − 5) and father was (2x − 5). The condition gives: 2x − 5 = 3(x − 5) = 3x − 15. Solving: 2x − 3x = −15 + 5, so −x = −10 and x = 10. The son is 10 years old now (and father is 20). Verify: five years ago son was 5, father was 15 = 3 × 5. ✓