With both the cold water and hot water faucets open it takes five minutes to fill a bathtub. the cold water facet alone takes 15 minutes to fill the tub. how long will it take to fill the tub with just the hot water faucet?

  7 1/2 minutes

Step-by-step explanation:

When the time with both faucets is a submultiple of the time with one faucet, you can use this sort of reasoning to figure the missing time.

Filling the tub in 5 minutes is equivalent to having 3 faucets open, each of which can fill the tub in 15 minutes. Since we know the cold water faucet can fill the tub in 15 minutes, the hot water faucet is equivalent to two open 15-minute faucets. That is, it can fill the tub in 15/2 = 7 1/2 minutes.

Perhaps a more conventional way to approach the problem is to consider "tubs per minute." The cold faucet runs at 1/15 tubs per minute, and the two faucets together run at 1/5 tub per minute. Then the hot runs at ...

  (1/5) - (1/15) = (3 -1)/15 = 2/15 . . . tubs per minute

So it will fill 2 tubs in 15 minutes, or 1 tub in 7 1/2 minutes.