Ravi can do a piece of work in 15 days while Prakash can do it in 30 days. In how many days will they finish it together?
Select.
A) 11 days  B) 9 days  C) 8 days  D) 12 days  E) 10 days

Rationale: "1 / 15 + 1 / 30 = 3 / 30 10 / 1 = 10 days answer : e"
Answer: E