both variables have a value of zero at the same time as one variable increases/decreases, the other variable increases/decreases in the same proportion ... For example, if n = 3, C = 20 × 3 ...

There is a direct proportion between two values when one is a multiple of the other. For example, \(1\text{cm} = 10\text{mm}.\) To convert \(\text{cm}\) to \(\text{mm}\), the multiplier is always 10.