Question 1017084
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Here's how to work it using "man days", which is "men times days".
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Twelve men can complete a work in 8 days. 
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So a job requires 12&#8729;8 or 96 man days.
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Three days after they started the work,...
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That was 12&#8729;3 or 36 of the required number of 96 man days.

So 96-36 or 60 more man days were needed.
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...3 more men joined them.
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So that made 15 men to complete the remaining 60 man days.

So 60 man days divided by 15 men gives 4 days.

Edwin</pre>