Difficulty: Medium
Correct Answer: 15 years
Explanation:
Introduction / Context:
Two symmetric time conditions (−5 and +5) precisely fix the present ages. Translate to equations and solve.
Given Data / Assumptions:
Concept / Approach:
Solve the pair by expressing M from both equations and equating them.
Step-by-Step Solution:
1) From past: M = 3m − 10.2) From future: M = 2m + 5.3) Equate: 3m − 10 = 2m + 5 ⇒ m = 15.4) Then M = 2m + 5 = 35.
Verification / Alternative check:
Five years ago: 30 vs 10 → 3×. In five years: 40 vs 20 → 2×. Both conditions hold.
Why Other Options Are Wrong:
They fail one of the two multiplicative checks upon substitution.
Common Pitfalls:
Mishandling the ±5 shift on both ages; both change simultaneously.
Final Answer:
15 years
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