If A, B and C together can finish a piece of work in 4 days. A alone in 12 days and B in 9 days, then C alone can do it in?
Answers:	A) 11	B) 17	C) 18	D) 19	E) 16

Rationale: "c = 1 / 4 - 1 / 12 – 1 / 9 = 1 / 18 = > 18 days ' answer : c"
Answer: C