Construct a linear model from a description

Let \(C\) represent total cost in $, and let \(d\) represent hire time in days. One unit of \(d\) represents 1 day

A trailer hire booking has an initial charge of $155. The charge then increases by $20 for each day

Only whole numbers of days are permitted

The linear model is claimed for \(0 \le d \le 16\)