German runways test a new ICE train between Munich and Berlin. The train runs to Berlin at a slow speed of 100 km/h. Back from Berlin goes faster. How quickly did the train have to go on a return trip so that the total average train speed for both journeys will be 200 km/h? (The task is from the customer magazine of German Railways ...)
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infinite speed is due that train goes first half at 100 km/h so it cannot achieve average speed 200 km/h, only if second half from Munich take 0 seconds (faster that speed of light)
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