Ages (past–future relation): In 10 years A will be twice as old as B was 10 years ago. A is now 9 years older than B. Find B's present age.

Introduction / Context:
This problem couples a forward shift in A's age with a backward reference to B's age, a common pattern in timeline-based age equations.

Given Data / Assumptions:

• A = B + 9 (A is currently 9 years older).
• In 10 years, A + 10 = 2 * (B − 10).

Concept / Approach:
Use substitution. Express A in terms of B from the age difference, substitute into the future–past relation, and solve a single linear equation for B.

Step-by-Step Solution:

Verification / Alternative check:
Check condition: In 10 years A = 58; B was 10 years ago = 29. Twice of 29 is 58 → satisfied.

Why Other Options Are Wrong:
Plugging them back violates the given equation.

Common Pitfalls:
Misplacing the 10-year shift across different persons; ensure “A + 10” pairs with “B − 10.”

Final Answer:
39 years