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?

Your selection is correct: 180 hours

The pool is apparently 18000 gallons (= 60 gal/h × 300 h). The combined flow from the two hoses is ...

... (40 gal/h) + (60 gal/h) = 100 gal/h

So, the time required with the two hoses is ...

... 18000 gal/(100 gal/h) = 180 h

only 10/60 isn't equivalent

hope this : )

3 is greater then both of those numbers