A can do a piece of work in 25 days. While B alone can do in 20 days. A started the work and was Joined by B after 10 days. The work lasted for
Number of hours A required do a piece of work : 25
Number of hours B required do a piece of work : 20
Work done by A in one hour: 
Work done by B in one hour: 
A works alone for 10 hours, so work completed by A in 10 hours : 10
= 
Work left = 
Work done by A and B together in one hour: 
= 
They can do the work together in 
hours.
But 
th of the work is done by both A and B
Time required to complete 
th of the work together by A and B : 
hours
Time taken to finish the work: 
= 
hours
13