Ravi can do a piece of work in 15 hours while Raman can do it in 12 hours. How long will both take to do it, working together?

Number of hours Ravi required do a piece of work : 15 hours

Number of hours Raman required do a piece of work : 12 hours

Work done by Ravi in one hour:

Work done by Raman in one hour:

Work done by Ravi and Raman together in one hour: = =

They can do the work together in hours = = 6 hours 40 min