Fixed charge and per-kilogram rate

A courier's charge \(C\) dollars for a parcel of mass \(m\) kg is modelled by \(C=c+dm\). A \(3\) kg parcel costs \(\$11\) and an \(8\) kg parcel costs \(\$21\).

1. Using the given information, write down two equations connecting \(c\) and \(d\).[2]
2. Solve the equations obtained in (i) to find \(c\) and \(d\).[3]
3. Find the mass \(k\) of a parcel that costs \(\$29\).[2]
4. State what the value of \(c\) represents in this model.[1]