By the definition, (average) acceleration is the difference in velocity over difference in time. In our case nobody cannot guarantee that the acceleration is constant during both episodes, therefore we use the notion of average acceleration.
Actually acceleration and velocity are vector quantities, in our case we suppose the fixed direction, therefore a scalar is sufficient.
The first difference in speed is 54 km/h - 0 = 54 km/h, and it is equal to 54*1000/3600= 54*5/18 = 15 m/s, so the acceleration is `15/15 = 1 (m/s^2).`
In the second episode there is negative acceleration, deceleration. It is equal to the same 15 m/s divided by 20 s, i.e. `0.75 m/s^2.`
The answer: there is acceleration of the magnitude `1 m/s^2` and deceleration of the magnitude `0.75 m/s^2.`
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