A formula is a mathematical rule or relationship. A formula uses letters to represent amounts that can be changed. These are called variables. For example, the formula to work out the area of a ...

A plumber charges £40 for a call out, and an extra £18 for every hour. How long did a job that cost £130 take? The formula for this plumber's charge is: \(T = £60 + £20 \times h\) where \(T\) is the ...