A formula is like a rule. It allows for an answer to be calculated when other information is known. For example, the area of a rectangle can be calculated when the length and width is known. The formula is shown with letters, numbers and arithmetical operators (signs).

The formula for the area of a triangle is:

A = `frac(1)(2)`*b**h*, where:

A is the area, *b* is the base and *h* is the height.

If the base is 15cm and the height is 26cm: replacing (or substituting) *b* with 15 and *h* with 26:

A = `frac(1)(2)`*bh*

A = `frac(1)(2)` x 15 x 26

A = 195 cm^{2}

Check carefully the units involved: if measurements are different units (for example, if a length was in metres and a width was in centimetres), make them all the same.

Emma is fund-raising and will be completing a long walk. One of her friends is going to give her £2, and a further 30p for each kilometre that she completes.

Write a formula for the contribution, £C, that will be donated for each kilometre, *k*, that she completes.

There will be a fixed contribution of £2. On top of that, there is £0.30 for each kilometre, remembering to change the 30p to pounds.

Answer: C = 2 + 0.3*k*

Emma completes a walk of 12 kilometres. How much did her friend contribute, according to the formula, above?

C = 2 + 0.3*k*

C = 2 + 0.3 x 12

C = 5.6. Remember to show the amount with two pence digits.

Answer: £5.60

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