Use a piecewise linear model
A water charge uses one rate for the first usage band and another rate after it
Let \(x\) represent water used (kL), and let \(y\) represent usage charge (dollars). The model is valid from 0 to 8 kL
The piecewise model is:
\[y = 23 + x,\quad 0 \le x < 4\]
\[y = 19 + 2x,\quad 4 \le x \le 8\]