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.