Relative Growth — “Adam is 20 and is 4 times as old as Mary. After how many years will Adam be twice Mary’s age, and what will Mary’s age be then?”

Introduction / Context:
We compare two future ages. Since Adam is currently 4 times Mary, we first get Mary’s present age, then find the time t when Adam becomes twice Mary’s age, and finally compute Mary’s age at that moment.

Given Data / Assumptions:

• Adam now = 20.
• Adam = 4 × Mary ⇒ Mary now = 5.
• In t years: Adam 20 + t; Mary 5 + t.
• Condition: 20 + t = 2(5 + t).

Concept / Approach:
Solve the linear equation in t, then evaluate Mary’s age at that time.

Step-by-Step Solution:

Verification / Alternative check:
At t = 10: Adam 30, Mary 15 ⇒ Adam is exactly twice Mary.

Why Other Options Are Wrong:
17/25/30/35 are not equal to 5 + 10 (the correct future age of Mary).

Common Pitfalls:
Confusing “twice” with “two more,” or forgetting to add t to both ages.

Final Answer:
15 years