A can do a piece of work in 10 days. He works at it for 4 days and then B finishes it in 9 days. In how many days can A and B together finish the work?
Options:
(A) 6
(B) 8
(C) 12
(D) 15
(E) 20 Let's think first. Stream of consciousness:
4/10+9/X = 1/x = 15
1/10+1/15 = 1/6 = 6 days
So, the answer is (A).