15. Working alone, R can complete a certain kind of job in 9 hours. R and S, working together at their respective rates, can complete one of these jobs in 6 hours. In how many hours can S, working alone, complete one of these jobs?
(A) 18(B) 12(C) 9
(D) 6(E) 3
答案选 A 求教 如何做出来的？谢谢
