there are 3 pipes A,B and C which can fill a tank in 10 hours, 15 hours and 20 hours respectively. All the three pipes are opened simultaneously. It was found that pipes A and B were supplying water at 4/5th of their normal rates for the first 2 hours after which they supplied at normal rate. Pipe C supplied water at 3/4th of its normal rate for the first 3 hours, after which it supplied at its normal rate. In how many hours would the tank be filled?

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