SOLUTION: An old machine can do a certain unit of work in 12 hours. To increase production, a more modern machine is put into operation. And both machines now take 4 hours to do the work. If

Question 825961: An old machine can do a certain unit of work in 12 hours. To increase production, a more modern machine is put into operation. And both machines now take 4 hours to do the work. If the old machine breaks down, how long will it take the new machine to do the work alone?
Answer by josmiceli(19441) (Show Source): You can put this solution on YOUR website!
Old machine's rate of working is:
( 1 unit of work ) / ( 12 hrs )
Rate of working with both machines:
( 1 unit of work ) / ( 4 hrs )
-----------------------------------
Let the rate of the new machine be:
( 1 unit of work ) / ( t hrs )
-----------------------------------
Add their rates of working to get their
rate of working together:

Multiply both sides by

The new machine takes 6 hrs to do the work alone
check:

OK
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