Why can't I just add the hours?
When the question asks for hours, it's asking for ordinary clock time, not "machine hours" counted separately for each device. If I start the four pumps at 9 am, then at 7:30 pm they will have emptied the pool. The question is simply how much clock time has passed from 9 am to 7:30 pm? 10.5 hr. We don't care how many different devices were running during that time, how many kilowatt-hours of energy were consumed, anything like that. We literally want the clock time; how much clock time does it take, running the four pumps, to empty the pool.