Question

The cut out lawn mowing company consists of two people: Tina and Bill. IF Tina cuts a lawn by herself she can do it in 4 hrs. If Bill cuts it it takes him an hour longer. How long would it take if they did it together?

Answer 1

There is a formula for this
Total Time = (Tina * Bill) / (Tina + Bill)
Total Time = (4*5) / (4+5)
Total Time = 20/9 hours

Still you might want a better explanation than that.

Answer 2

Tina's rate = .25/hour (meaning .25 of the lawn is cut per hour).
Bill's rate = .20/hour
So, in one hour, (.25 + .20) part of the lawn is cut.
or .45 parts per hour

Rate for both Tina and Bill = .45 lawn/one hour
We need to know the time for the total lawn so the total time would be hours/lawn.
To get hours per lawn, we take the reciprocal of .45/1 hour OR
1/.45 which equals
2.222222 hours
(which equals 20/9, my previous answer).