Originally Posted by

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

so start with $\displaystyle C = mw + b$

where $\displaystyle C$ is the charge, $\displaystyle m$ is the slope (the charge per cubic foot of water used), and $\displaystyle b$ is the y-intercept (the fixed charge), and $\displaystyle w$ is the amount of water used in cubic feet

now we need to find $\displaystyle m$ and $\displaystyle b$. once we have those, we can answer the questions.

we are given two points on this line: $\displaystyle (w_1,C_1) = (1000, 72)$ and $\displaystyle (w_2,C_2) = (1600, 108)$

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