Difficulty: Hard
Correct Answer: 12 years
Explanation:
Introduction / Context:This multi-snapshot problem provides two present/future ratios and an earlier marriage-time ratio. First determine the actual present ages from the now/after-4-years relation; then walk back to the marriage-time ratio to compute the elapsed years since marriage.
Given Data / Assumptions:
Concept / Approach:First solve for k using the “after 4 years” ratio. Then substitute present values into the marriage-time ratio to solve for t, the time elapsed since marriage.
Step-by-Step Solution:
From after-4-years: 7(4k + 4) = 9(3k + 4) ⇒ 28k + 28 = 27k + 36 ⇒ k = 8. Thus M = 32 and W = 24. Marriage-time condition: (32 − t) : (24 − t) = 5 : 3 ⇒ 3(32 − t) = 5(24 − t) ⇒ 96 − 3t = 120 − 5t ⇒ 2t = 24 ⇒ t = 12.Verification / Alternative check:After 4 years: 36 : 28 reduces to 9 : 7 (✓). Marriage-time ratio 20 : 12 is 5 : 3 (✓).
Why Other Options Are Wrong:Any other t fails at least one of the ratio checks when ages are rolled back or forward appropriately.
Common Pitfalls:Using the same k at marriage time (ages do not scale; they shift by t), or forgetting to subtract t from both spouses.
Final Answer:12 years
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