Difficulty: Medium
Correct Answer: 6
Explanation:
Introduction / Context:
Use the women-only completion time to define the total job, then determine a child’s rate by balancing the mixed team against the known total. Finally, scale to nine children.
Given Data / Assumptions:
Concept / Approach:
From (3w + 18c)*2 = 18w, solve for c in terms of w. Then compute time for 9 children = W / (9c).
Step-by-Step Solution:
Verification / Alternative check:
Back-substitute c = w/3 into the mixed team to confirm the total matches W.
Why Other Options Are Wrong:
They conflict with the derived equivalence c = w/3.
Common Pitfalls:
Forgetting to multiply by the number of days when forming equations; dropping the factor 2.
Final Answer:
6
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