Sometimes one value can be expressed as an expression of another value ā these are called algebraic formulae.

The advert says that the cost of a party is plus per person, so what would be the cost of a party for ?

The cost would be .

- Question
What would be the cost of a party for ?

How do you work out the cost if you don't know the number of people who will attend the party?

Call the number of people ' '.

The cost for people is .

The cost for people is .

So the cost for people is .

With this formula you can work out the cost of a party for any number of people.

Have a go at the next question to see if you can use in another expression.

- Question
Alexa is years old. How old will she be in years' time?

In years' time her age will have increased by . So her age will be more than it is now.

Her age will be:

Check your answer. In one year's time she will be . If we put in for the value of we get . So that is correct.

In ten years' time she will be , and putting gives us:

So the formula is correct.