Run, mathematician, RUN!

Algebra Level 2

One day at the gym I walked onto the treadmill to begin my training. I planned to run for 20 minutes, scaling my first 10 minutes of training like this:

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)


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