A, B and C can do a piece of work in 15, 12 and 20 days respectively. They started the work together, but C left after 2 days. In how many days will the remaining work be completed by A and B?
Number of days A required do a piece of work : 15 days
Number of days B required do a piece of work : 12 days
Number of days C required do a piece of work : 20 days
Work done by A in one day: 
Work done by B in one day: 
Work done by C in one day: 
Work done by A, B and C together in one day: 
= 
They can do the work together in 5 days.
A, B and C worked together for 2 days, so work completed by A, B and C in 2 days : 2
= 
Work left = 
Work done by A and B together in one day: 
= 
They can do the work together in 
days.
Number of days taken by A and B to complete the left over work : 
= 4 ( here 
is days required by A and B to complete the work).
Time taken to finish the left over work: 4 days.