Dave can mow his lawn in 20 minutes less time than with a power mower that with his hand mower. One day, the power mower broke 15 minutes after he started mowing and he took 25 minutes more time to complete the job with the hand mower. How many minutes does Dave take to mow the lawn with the power mower?

The book tells me to use this:

rate of workxtime worked = part of job completed

Who can clarify this question for me?