The answer would be about 22.1 minutes. The reason for this is because Lisa can shovel 1/60 of the driveway in 1 minute. Jim can shovel 1/35 of it in one minute. If you add that up, 1/60 + 1/35 and by using common denominators you'd get 7/420 + 12/420 which is 19/420. If you flip that, 420/19 is about 22.1.