Question 714440
    An oil tank has two inlet pipes and one outlet pipe. One inlet pipe can fill the tank in 12 hours, and the other inlet pipe can fill the tank in 20 hours. The outlet pipe can empty the tank in 10 hours. How long would it take to fill the tank with all three pipes open?
Let x = time (hours) it takes to fill
then
x(1/12 + 1/20 - 1/10) = 1
multiply both sides by 120: common denominator
x(10 + 6 - 12) = 120
x(16 - 12) = 120
x(4) = 120
x = 30 hours