Average acceleration:-When the velocity of an object is changing continuously with different rate as like

a car from rest cover 20m/s speed in 5sec ,,then 30 m/s in 1s,then 80m/s in 8s

u can see here ,,the rate if Changing the velocity is different,i.e there is different acceleration for different time

the actual formula for average acceleration is

The average acceleration will be zero,if V2-V1=0

this can be possible when V2=V2

Instantaneous acceleration:-It is the Acceleration at any instant or at any moment of time

It is eqaul to ∆V/∆t

Example in which instantaneous acceleration is not zero,,but average acceleration is zero

consider a car starts moving with velocity 10m/s.The velocity of car is increasing continuously upto time 2min,,,then due to crowdy area it starts to decelerate and its velocity again became 10m/s in time of 3min

in this case average acceleration=10-10/180-0=0

instantaneous acceleration is the acceleration at any instant,,,

since at every moment of time the velocity of car continuously changes,,so ∆v will Never be zero

hence

instantaneous acceleration will never be zero

Answer:

Consider a simple example when we start a vehicle, initially we have zero velocity but have non zero acceleration ( that is why vehicle will start moving). If we drop a ball from some height above the Earth surface the velocity at that instant is zero but acceleration is non zero(gravity) which acts downward.