A tank is fitted with two inlet pipes A and B and an outlet pipe C. Pipe A can fill the empty tank in 12 minutes. While pipe B alone can fill it in 18 minutes. Pipe C can empty the full tank in 11 1/4 minutes. If all three pipes are opened simultaneously, in what time will the empty tank be filled?
Part of the tank filled in 1 minute when all three pipes are opened
= $$\frac{1}{12} + \frac{1}{18} - \frac{\frac{1}{45}}{4}$$
= $$\frac{1}{12} + \frac{1}{18} - \frac{4}{45}$$
= $$\frac{9}{180} = \frac{1}{20}$$
=> Required time = 20 minutes
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