To calculate the rate of change, one must divide the total change by the amount of time that it took, in this case years. One can find the total change by subtracting the two quantities, $15,200 and $8,000. This gives a rate of change of $7,200 per four years. To find the change per year, one must do what I said in the first sentence, divide the total change by the amount of time. Doing this, one gets $1,800 changed per year, assuming there was a constant rate of change.
