A can complete a piece of work in 18 days and B can do it in 12 days. They start the work together, but after 4 days, A leaves the job. In how many days was the whole work completed?

The combined rate of A and B is (1/18) plus (1/12), which equals 5/36 of the work per day. In 4 days, they complete 20/36 of the work, leaving 16/36 of the work. B alone completes this remaining work at a rate of 1/12 per day, taking (16/36) divided by (1/12), which equals 16/3 days. The total time is 4 days plus 16/3 days, which equals 28/3 days or 9 (1/3) days.

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