# A marathon

**Algebra**Level pending

He runs the first 10 km at an average speed of 5 m/s, the next 10 km at an average speed of 4.5 m/s and the third 10 km at an average speed of 4 m/s.

How much time ( to the nearest second )does he have to run the last 12 km if he wants to equal his best time?

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