Because we are already given the unit rate in money per pound (money/pound), and we must determine the quantity, we must rearange the equation for rate to solve for the quantity we are looking for. Since in this problem, the rate = change in money / change in pounds. This done by first subtracting the money on the gift card from the money remaining to determine the change in money. Then divide this by the rate of $7.81 (money per pound) to cancel out the money and get the pounds of coffee.
So:
rate (in this case, $7.81 per pound) = change in money / change in pounds →
change in pounds = change in money / rate ($7.81 per pound)
