1. Let t denote the time (measured in minutes) you need to mow the lawn by hand. Let L denote the area of the lawn.

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. Thereforet > 20.

3.

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.)