A and B together can do a work in 6 days. If A alone can do it in 10 days. In how many days can B alone do it?	Select
A) 10	B) 15	C) 77	D) 55	E) 21

Rationale: "1 / 6 – 1 / 10 = 1 / 15 = > 15 answer : b"
Answer: B