If A can do a piece of work in 12 days and B can do it in 18 days, how long will it take for both A and B to complete the work together?

Answer: A. 7.2 days
A's rate is 1/12 and B's is 1/18. Together, they work at a rate of 1/12 + 1/18 = 5/36. Therefore, they will complete the work in 36/5 = 7.2 days.
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