Answer: it takes Lisa and Jim 19.6785 hours to shovel the driveway
Explanation:
It takes Lisa 45 minutes to shovel snow from her driveway. This means that her unit rate of working in terms of time is 1/45
It takes Jim 35 minutes to shovel snow from the same driveway. This means that his unit rate of working in terms of time is 1/35
If they work together, it would take a shorter time to complete the work.
This means that they would work simultaneously and their unit rates are additive.
The combined unit rates will be
1/45 + 1/35 = (35+45)/1575
= 80/1575
Assuming that it takes a combined time of t hours to finish the work, then the combined unit rate of working will be 1/t. Therefore,
80/1575 = 1/t
t = 1575/80 = 19.6875 hours