Age puzzle using present age difference: “I was your present age when you were born.” Father is 36 now—boy’s age five years ago?

Introduction / Context:
Classic age-word problem: a statement relates the father’s age at the son’s birth to the son’s present age. Translate into an equation using the constant age difference between parent and child.

Given Data / Assumptions:

• Father’s current age = 36.
• Let the son’s current age = s.
• Father says: “I was of your present age when you were born.”
• Find the son’s age 5 years ago = s − 5.

Concept / Approach:
At the son’s birth, the father’s age equals the son’s current age s. The present age difference equals the father’s age at that time: father_now − son_now = s. Solve for s directly.

Step-by-Step Solution:

Verification / Alternative check:
If the son is 18 now, then 18 years ago (at son’s birth) the father was 18 (matching the son’s current age). This confirms the condition.

Why Other Options Are Wrong:

• 15, 17, 20: These correspond to current ages 20, 22, 25 respectively, which contradict the father’s statement when checked.

Common Pitfalls:
Interpreting the quote as a sum rather than a difference, or forgetting that the parent-child age gap is constant.

Final Answer:
13