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A road from A to B is 11 km long, first goes uphill, then crosses a plain, and then goes downhill. A person walking from A to B covered this road in 2 hours 54 minutes, and the return journey took him 3 hours 6 minutes. His speed uphill is 3 km/h, on the plain is 4 km/h and downhill is 5 km/h. What is the length of the plain part of the journey?