Sometimes in mathematics we like to use letters to represent ... rules or conditions we are given – hence we can call them variables. There is only one answer to the value of the letter a.

The constant of proportionality is 20, so C = 20n. For example, if n = 3, C = 20 × 3 If C = 20 × 3, the cost is 60p. Two variables are directly proportional when as one variable increases ...