A and B together can complete a work in 12 days. A alone can complete it in 20 days. If B does the work only for half a day daily, then in how many days A and B together will complete the work?

Question: A and B together can complete a work in 12 days. A alone can complete it in 20 days. If B does the work only for half a day daily, then in how many days A and B together will complete the work?

Solution:
B's 1 day's work = 1/12 - 1/20 = 1/30
If B does the work half a day, then he will do = 1/30 ÷ 2 = 1/60

Now, (A + B)'s 1 day's work = 1/20 + 1/60 = 1/15

So, A and B together will complete the work in 15 days.