If I can make it around a running track once by going 10 km/h, what speed should I be running a second lap to have an average speed of 20 km/h for the two laps ?
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This is impossible, I need to be infinitely fast.
Imagine a 10km track. It will take me 1 hour to make a lap at 10km/h. With a second lap, it becomes 20km. To have a 20km/h average speed, I need to do those 2 laps in 1 hour.
But I already spent 1 hour, so I should be infinitely fast for the second lap, which is impossible.