A and B together can do a piece of work in 10 days. B alone can finish it in 20 days. In how many days can A alone finish the work ?
Choose the correct answer:
A) 20  B) 10  C) 15  D) 5  E) 7

Rationale: time taken by a to finish the work = xy / ( y - x ) = 10 x 20 / ( 20 - 10 ) = 200 / 10 = 20 days answer : a
Answer: A