When does the instantaneous rate of REACTION equal the average rate of REACTION?
They are equal at the point where the slopes are equal. For example, the average rxn rate is a constant slope on a plot such as mol concentartion C (gmol/L) vs time (sec). An instantaneous rxn rate is the derivative, dC/dt at some time, t. Since the derivative is just the unique tangent or slope at that point, t, you can solve for this point by setting the derivative equal to the average slope.