A winery has a vat with two pipes leading to it. The inlet pipe can fill the vat in 7 hours, while the outlet pipe can empty it in 9 hours. How long will it take to fill the vat if both pipes are left open?
1/7 + 1/-9 = 1/x solve for x = 31.5 hours
[9 / (9 - 7)] x 7 =31.5 hours to fill the Vat.