To find the rate of change in the number of gallons in the wading pool after each hour, we can use the formula for the slope between two points:
\[ \text{slope} = \frac{y_2 - y_1}{x_2 - x_1} \]
Using the points \((x_1, y_1) = (1, 160)\) and \((x_2, y_2) = (3, 80)\):
\[ y_1 = 160, \quad y_2 = 80, \quad x_1 = 1, \quad x_2 = 3 \]
Plugging in the values:
\[ \text{slope} = \frac{80 - 160}{3 - 1} = \frac{-80}{2} = -40 \]
Therefore, the rate of change in the number of gallons in the pool after each hour has passed is:
\[ \text{-40 gal./hr.} \]
So the correct response is:
−40 gal./hr.