it seems obvious that there is a linear relationship here between the charge and the amount of water used.

so start with

where is the charge, is the slope (the charge per cubic foot of water used), and is the y-intercept (the fixed charge), and is the amount of water used in cubic feet

now we need to find and . once we have those, we can answer the questions.

we are given two points on this line: and

we can use these two points to find the slope, and then use that to find the y-intercept