A can do a piece of work in 10 days and B alone can do it in 15 days. How much time will both take to finish the work ?  Choose the correct choice from the following answers
A) 2 days B) 7 days C) 6 days D) 9 days E) 3 days

Rationale: "c 6 days time taken to finish the job = xy / ( x + y ) = 10 x 15 / ( 10 + 15 ) = 150 / 25 = 6 days"
Answer: C