A is twice as fast as B. If B alone can do a piece of work in 18 days, in what time can A and B together complete the work?	Choose the correct choice:
A) 10	B) 16	C) 18	D) 6	E) 12

Rationale: "a can do the work in 18 / 2 i . e . , 9 days . a and b ' s one day ' s work = 1 / 9 + 1 / 18 = ( 2 + 1 ) / 18 = 1 / 6 so a and b together can do the work in 6 days . answer : d"
Answer: D