Difficulty: Easy
Correct Answer: 60 years
Explanation:
Introduction: The life-stage breakdown forms a fractional equation in the total age. The fixed 13-year old-age segment plus fractional parts of the same total must equal the entire lifespan.
Given Data / Assumptions:
Concept / Approach: Add the fractions and 13, set equal to x, then solve for x using a common denominator to combine fractions exactly.
Step-by-Step Solution:
x/4 + x/5 + x/3 + 13 = x.Common denominator 60: (15x + 12x + 20x)/60 + 13 = x → 47x/60 + 13 = x.x − 47x/60 = 13 → (13x/60) = 13 → x = 60.Verification / Alternative check: Check segments: 15, 12, and 20 years sum to 47 years; plus 13 equals 60, consistent.
Why Other Options Are Wrong: 70, 80, 36, 66 do not satisfy the fractional partition equation.
Common Pitfalls: Arithmetic errors with fraction addition; ensure exact common denominator.
Final Answer: 60 years
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