A can do a piece of work in 12 days, B can do it in 15 days, and C can do it in 20 days. How long will they take to complete the work together?

Answer: B. 6 days
The combined work rate is 1/12 + 1/15 + 1/20 = 1/4. Therefore, they will complete the work together in 4 days.
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