Susan can wax her car in 2 hr(s). When she works together with Helga, they can wax the car in thirty min(s). How long would it take Helga, working by herself, t0 wax the car?
Helga can wax the car by herself in 40 minutes.
Working together, Susan will wax 1/4 of the car in 30 minutes. Helga will wax the other 3/4 of the car in 30 minutes.