Using the points (1, 160) and (3, 80), we can calculate the rate of change in the number of gallons in the pool as:
Rate of change = (Change in y) / (Change in x)
Rate of change = (80 - 160) / (3 - 1)
Rate of change = -80 / 2
Rate of change = -40
Therefore, the rate of change in the number of gallons in the pool after each hour has passed is -40 gal/hr.