Answer: It's 12.5 hours.
Explanation:
12.5 hours is correct. You are increasing the number of men.
5 men × 20 hours = 100 man hours
100 man hours ÷ 8 men = 100 /8 = 12.5 hours
So 20 / 5 = 4 hours per person and then you need to do 20 / 8
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