Run, mathematician, RUN!
3 minutes at 6 km/h
2 minutes at 7.5 km/h
3 minutes at 8 km/h
2 minutes at 9 km/h
After this I was a bit tired and I slowed the speed down to 5 km/h. At a certain point, I decided to start running again, this time at 10 km/h until the end of the training.
At 20 minutes sharp, I reached exactly 2.5 km.
At which time, in seconds from the beginning of the training, did I raise the speed to 10 km/h?
(Don't consider the acceleration, let's just state that the speed is automatically and exactly shifted)