Awinery has a vat with two pipes leading to it. the inlet pipe can fill the vat 7 hours, while the outlet pipe can empty it in 10 hours. how long will it take to fill the vat if both pipes are left open?

Lets say, for ease, that the vat can hold a total of 70 gallons (or whatever you would like to use.) Use whatever number you want, I just picked this because it gives us a lot of clean numbers. 

Now, if the inlet can fill it in 7 hours, that means that it is adding 10 gallons per hour. (70 gal/7 hours = 10 gal/hr)

For the outlet, use the same process, and you find that it drains the vat at 7 gallons per hour. 

So, if you subtract the outlet from the inlet, you get 10 - 7 = 3 gallons per hour added. 

Now just divide the size of the vat by that number, and you find your answer. 

70 gallons / 3 gallons per hour = 23 1/3 hours.