Problems on Ages — “A was born 5 years before B; B is 4 years older than C and 3 years younger than D. If A is now 17, how old is D?”

Introduction / Context:
This is a linear “age chain” with several offsets. Anchoring the chain at the known present age (A) and walking step-by-step to each related person keeps the arithmetic simple and accurate.

Given Data / Assumptions:

• A now = 17.
• A was born 5 years before B ⇒ A = B + 5.
• B is 4 years older than C ⇒ B = C + 4.
• B is 3 years younger than D ⇒ D = B + 3.

Concept / Approach:
From A = B + 5 we first find B, then C from B = C + 4, and finally D from D = B + 3.

Step-by-Step Solution:

Verification / Alternative check:
Check all differences: A (17) is indeed 5 older than B (12); B (12) is 4 older than C (8); D (15) is 3 older than B (12). All constraints satisfied.

Why Other Options Are Wrong:
8/12/19/21 conflict with at least one of the stated pairwise differences when cross-verified against A = 17.

Common Pitfalls:
Reversing “older than/younger than”; attempting to jump directly from A to D and missing the intermediate B relation.

Final Answer:
15 years