To find the rate of change in the cost of gas per gallon, you can use the formula for the slope (m) of a line, which is given by:
\[ m = \frac{y_2 - y_1}{x_2 - x_1} \]
In this case, the two points given are \( (4, 15) \) and \( (8, 30) \). Here, \( (x_1, y_1) = (4, 15) \) and \( (x_2, y_2) = (8, 30) \).
Now, plug the coordinates into the formula:
\[ m = \frac{30 - 15}{8 - 4} = \frac{15}{4} = 3.75 \]
So, the rate of change in the cost of the gas as each gallon is pumped is:
\[ 3.75 \text{ dollars per gallon} \]