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Average velocity is calculated over a period of time. For instance, let's say the initial position is zero meters. The final position is twenty meters. If it took five seconds to travel twenty meters, then the average velocity is four meters per second. 
Instantaneous velocity is calculated at an instance of time. An example would be calculating the velocity at twenty-one meters at a time of exactly three seconds. The instantaneous velocity would be seven meters per second. 
In both cases, derivatives and/or limits can be used to make velocity calculations.
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average velocity is the ratio change of position with respect to time written and 
 instantaneous velocity is at particular moment is calculated by substituting the corresponding time variable's value in the first time derivative of the displacement equation