A can finish a piece of work in 12 hours while B can finish it in 15 hours. How long will both take to finish it, working together?
Number of hours A required do a piece of work: 12
Number of hours B required do a piece of work : 15
Work done by A in one hour: 
Work done by B in one hour: 
Work done by A and B together in one hour: 
= 
= 
They can do the work together in 
hours = 6
hours.
7