Hello! Your answer would be 13 days.
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If there are 2 times more men, then 2 times as much work will get done each day. This would mean that the work would be done 2 times faster, so it would take half as long. Therefore, half of the time it would take five men to build it, 26 days, would be how long it would take 10 men to build it. Half of 26 is 13, so therefore, it would take 13 days.
Essentially, 2 times as many men, half the time it takes.
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Hope this helps!
