Give a quick stream of consciousness before answering the following 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?
Options:
(A) 11 days
(B) 77 days
(C) 15 days
(D) 18 days
(E) 19 days
B's 1 day work = (1/12 - 1/20) = 1/30
Now, (A + B)'s 1 day work = (1/20 + 1/60) = 1/15
So, A and B together will complete the work in 15 days.
The answer: (C).