An object is thrown upwards at a speed of 20 m/s. How many seconds will it take to come back down?

The time to reach its highest point is just 2 seconds --- the time needed to lose 20 m/s of speed.

The downward trajectory is exactly the the upward one played backwards and will take the same amount of time --- 2 seconds.

Total time = 2 s + 2 s = 4 s.

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