Difficulty: Medium
Correct Answer: 38 years
Explanation:
Introduction / Context:
This classic ages problem mixes number theory with linear reasoning. We are told that the present age x satisfies two near-neighbor conditions: last year (x − 1) was a perfect square, and next year (x + 1) will be a perfect cube. We then ask: counting forward from today, after next year's cube, how long until his age is once again a perfect cube?
Given Data / Assumptions:
Concept / Approach:
Subtract the two conditions: (x + 1) − (x − 1) = m^3 − n^2 ⇒ 2 = m^3 − n^2. Test small cubes until the right-hand side equals 2. With m = 3, m^3 = 27 and n^2 = 25, satisfying 27 − 25 = 2. Hence x + 1 = 27 ⇒ x = 26 and x − 1 = 25 = 5^2 (consistent).
Step-by-Step Solution:
1) Solve m^3 − n^2 = 2 ⇒ m = 3, n = 5.2) Current age x = 26; next year x + 1 = 27 = 3^3 (first cube).3) The next cube after 27 is 64 = 4^3.4) Waiting time from today to reach 64 is 64 − 26 = 38 years.
Verification / Alternative check:
Check neighbor conditions around x = 26: last year 25 is 5^2; next year 27 is 3^3. The next cube after 27 is indeed 64, so 38 years is correct.
Why Other Options Are Wrong:
Common Pitfalls:
Answering “1 year” by misreading “again” as the first cube (next year). The problem asks for the next cube after the immediate one.
Final Answer:
38 years
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