You divide a change in velocity by the time it takes for that change.
average acceleration is the average of the acceleration of a body in its entire motion where as instantaneous acceleration is the rate of change of velocity at an instant. it may be a function of time or velocity or displacement.
Average acceleration is the average of the accelerations acquired in the whole journey by a body while instantaneous acceleration is the acceleration of the body at any particular instant of time.
You have a contradiction in your question. Instantaneous acceleration is the acceleration at a certain moment in time. Average acceleration is the average over a time interval.
Acceleration can be found by taking the derivative of a function for velocity, as acceleration is the change in velocity with respect to time.
Average acceleration will be equal to instantaneous acceleration when an object has an uniform acceleration throughout its motion. Example : A car accelerating at 1m/s2 uniformly in a straight line.
Average acceleration = Change in speed/time so Time = Change in speed/Average acceleration
The average acceleration can be obtained by finding the slope of the graph. The instantaneous acceleration is found by drawing a tangent to a particular point on the graph (instant) and finding the slope of than tangent.
Average Acceleration = V/t = Vf-Vi / Tf-Ti
All acceleration in real life is an average figure = velocity increase / time
Average speed = Distance travelled/time to travel the distance . Average acceleration = Change of speed/time for the change .
Acceleration = (change in speed) divided by (time for the change)
That's true throughout any period of time during which the acceleration is constant.