A and B working together could mow a field in 28days and with the help of C they could have mowed it in 21 days. How long would C take by himself?
Choose the correct choice from the following answers
A) 68 days B) 84 days C) 90 days D) 72 days E) 50 days

Rationale: required answer = 28 * 21 / 28 - 21 = 84 days answer is b
Answer: B