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A cistern can be filled by two taps A and B coming from the same main pipeline in 20 and 30 minutes respectively. Both the taps are opened at the same time but due to partial closing of a valve in the main pipeline, tap A was supplying only 4/5th of its capacity and tap B, 5/6th of its capacity. After some time, the valve in the main pipeline was opened fully thereby enabling the two taps to supply at full capacity. It took another 5 minutes to fill the cistern completely. How long (approx.) was it before the valve on the main pipeline opened?