Grade 12 mathematics is, more than anything, the study of CHANGE — and every measure of change in this course is a quotient. The first one has a name you already own: the average rate of change, $m = \dfrac{y_2 - y_1}{x_2 - x_1}$. Read aloud: m equals y-two minus y-one, over x-two minus x-one. Letter by letter — $y$ is the responding quantity (litres in the tank, dollars on the invoice, metres off the ground), $x$ is the driving quantity you measure it against (minutes, kilograms, seconds), and the subscripts are a reading convention, not an arithmetic one: 1 is the first reading, 2 is the second. $m$ is what comes out — how much y moves for every one unit x moves.

Before any of that, though, the units do the sorting for you. A rate always wears a per — m/s, $/kg, °C/h, two units deep with a slash between them. An amount wears one plain unit and nothing else. A duration wears a clock unit alone, and its job is to be the run, the thing a rate is measured against. Sort those three on sight and half of this chapter has already been done for you.