X and Y can do a piece of work in 20 days and 12 days respectively. X started the work alone, and then after 4 days, Y joined him until the completion of the work. How long did the work last?

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X and Y can do a piece of work in 20 days and 12 days respectively. X started the work alone, and then after 4 days, Y joined him until the completion of the work. How long did the work last? Answer: 10 days