Question

a winery 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?

Answer 1

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.