R and S together can plough a field in 10 hours but by himself R requires 15 hours. How long would S take to plough the same field?  Choose the correct answer.
A) 24 hours	B) 5 hours	C) 10 hours	D) 15 hours	E) 20 hours

Rationale: f r and s together can do a piece of work in x days and r alone can do the same work in y days , then s alone can do the same work in x y / y â € “ x days . the no . of hours required by s = 10 ã — 20 / 20 â € “ 10 = 200 / 10 = 20 hours e
Answer: E