Hi

Here is my problem:

If the hot water faucet is left on alone, it takes 20 minutes to fill the bathtub. It takes only 15 minutes to fill the bathtub if the cold water faucet is left on alone. It takes 12 minutes to fill the bathtub if both faucets are left on and the drain is open. How long will it take to drain a full bathtub?

The answer is 30 minutes, but I don't know how to get that. Any help would be appreciated. Thanks.