Numerical answer
A can complete a job in 12 days and B can complete it in 18 days. They work together for 4 days, after which A leaves and B completes the remaining work alone. How many more days does B take to finish the job after A leaves?
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Answer: 8
Combined 1-day work of A and B = 1/12 + 1/18 = 3/36 + 2/36 = 5/36. In 4 days, they complete 4 × 5/36 = 20/36 = 5/9 of the work. Remaining work = 1 − 5/9 = 4/9. B's 1-day rate = 1/18, so days needed by B alone = (4/9) ÷ (1/18) = (4/9) × 18 = 8 days.
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