Then (t-20) is the time you need when you use a power mower.
2. In one minute the hand mower has done of the lawn and
in one minute the power mower has done of the lawn. Therefore t > 20.
Divide through by L. Multiply by the denominators to get rid of the fractions:
Solve for t.
I've got t = 10 or t = 50. So the only possible solution is t = 50. (see #2.)