If A can do a piece of work in 15 days and B can do it in 10 days, how long will it take for both A and B to complete the work together?

Answer: A. 6 days
A's work rate is 1/15 and B's work rate is 1/10. Together, they work at a rate of 1/15 + 1/10 = 1/6, thus taking 6 days to complete the work.
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