Question

How long will it take to fill the pool if two hoses are used, one that fills at a rate of 40 gallons per hour and one that fills at a rate of 60 gallons per hour?

Answer 1

One data is missing. You need the volume of the pool or some data that permit you to calculate this volume.

The same statement given by you is part of a problem where you know a series of data that shows the time required to fill the pool at different flow rates:

flow rate              time
60 gal/h               300 h => 60 gal/h * 300 h = 18,000 gal
45 gal/h               400 h => 45 gal/h * 400 h = 18,000 gal
36 gal/h               500 h => 36 gal/h * 500 h = 18,000 gal
30 gal/h               600 h => 30 gal/h * 600 h = 18,000 gal


So for, this pool to calculated (several times) that the volume is 18,000 gal.

Now you have two hoses one with a flow rate of 40 gal / h and the other with a  flow rate of 60 gal/h.

The total flow rate is the sum of the two flow rates"

total flow rate = 40 gal/h + 60 gal/h = 100 gal/h

And you just must divide the volume of the pool (18,000 gal) by the total flow rate (100 gal/h) to get the time to fill the pool:

time = volume / flow rate = 18,000 gal / 100 gal/h = 180 h.

Answer: 180h