You are only allowed to ask one question at a time so I edited this down accordingly. The units are the same for both because they are ultimately both measuring the same thing, a rate. The instantaneous rate of change for something is the rate of change at a particular instant in time. The average rate of change of something is the rate averaged out over a given time interval. For example, say you are measuring the rate that a chemical is being produced in a chemical reaction. Whether you are measuring the rate at a particular moment in time (say at t=2 s) or over a particular time interval (say from 0 s to 15 s), you are still measuring the amount of product formed per unit time (moles per liter per second is common).
so what is the answer for this question for a given set of data two students determined different average reaction rates. If neither student made an error in their calculations, account for the difference in their reaction rates.