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:  A) 68days  B) 84days  C) 90days  D) 72days  E) 50days
Required answer = 28*21 / 28-21 = 84days   Answer is 84days
<result>B</result>
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?