A can do a work in 8 days and B in 12 days. If they work on alternate days, starting with A, in how many days will the work be finished?
Correct answer: C. 9.5 days
Two days (A then B) = 1/8 + 1/12 = 5/24. After 8 days (4 pairs) 20/24 is done, leaving 4/24 = 1/6. Day 9 is A's: he does 1/8 = 3/24, leaving 1/24, which B finishes in half a day (1/24 ÷ 1/12). Total 9.5 days.
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