Q: If A can complete a piece of work in 7 days, working 9 hours per day, and B can complete the same work in 6 days, working 7 hours per day, how long will it take for them to complete the work if they work together 8 hours per day?

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Answer: C

Explanation:

A can complete the work in 7 days, working 9 hours per day,
so he takes a total of 7 * 9 = 63 hours to complete the work.
Similarly, B takes a total of 6 * 7 = 42 hours to complete the work.
A's 1 hour's work is (1/63) of the total work, and B's 1 hour's work is (1/42) of the total work.
When they work together, their combined 1 hour's work is (1/63) + (1/42) = (5/126) of the total work.
Since they are working 8 hours per day, it will take them 126 / (5 * 8) = 3 days to complete the work.

Q: If A can complete a piece of work in 7 days, working 9 hours per day, and B can complete the same work in 6 days, working 7 hours per day, how long will it take for them to complete the work if they work together 8 hours per day?

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