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?

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.

180h