Q: In this task, you need to answer the given multiple-choice question on the physics. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.
Problem: a and b working together could mow a field in 28 days and with the help of c they could have mowed it in 21 days . how long would c take by himself ?
Options: a ) 68 days , b ) 84 days , c ) 90 days , d ) 72 days , e ) 50 days
A:
b