When Mike uses a riding mower, it takes him 3 hours to mow his lawn. When he uses a push mower, it t

When Mike uses a riding mower, it takes him 3 hours to mow his lawn. When he uses a push mower, it takes him 6 hours to mow the lawn. (His sister also can mow the lawn with the push mower in 6 hours.). Mike wanted to get the lawn mowed as quickly as possible, so he paid his sister $10 to mow with the push mower while he used the riding mower. How long will it take Mike an his sister to mow the lawn if they worked together?

Mike can mow 1/3 of the lawn in an hour.
Mike's sister can mow 1/6 the lawn in an hour.
together, they can mow 1/3 + 1/6 = 1/2 of the lawn in one hour.

Which means it would take 2 hours (2 * 1/2) = 1 to mow the full lawn.