Three pipes, A, B, & C are attached to a tank. A & B can fill it in 20 & 30 minu

Three pipes, A, B, & C are attached to a tank. A & B can fill it in 20 & 30 minutes respectively while C can empty it in 15 minutes. If A, B & C are kept open successively for 1 minute each, how soon will the tank be filled?

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