Pipe X can fill a tank in 20 hours and Pipe Yin 30 hours. Both are opened, but Pipe X is closedafter some time and the tank is full in 18 hours.After how much time was X closed?

Using the one unit work method, the filling rates of pipes X and Y are 1/20 and 1/30 of the tank per hour, respectively. Let pipe X be open for T hours; since the tank is full in 18 hours and Y works the entire time, the equation is T/20 + 18/30 = 1. Solving for T gives T/20 + 3/5 = 1, which means T/20 = 2/5, so T equals 8 hours. The result is 8 hours.

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