Substitute Values into Simple Formulae

A formula tells you how to work something out based on what you know already.

If you know the length of the side of a square, how would you work out its perimeter?

The answer is to multiply it by 4 because a square has 4 sides of equal length.

We can use a formula to state this, but first we must say what the letters stand for.

s = side length of square

P = perimeter of square

The formula will be

P = 4s

which means P = 4 × s

If s = 5,

P = 4 × 5 = 20

If s = 15,

P = 4 × 15 = 60

Example

Mum uses the following formula to work out our pocket money.

P = A × 30 + 100

where P stands for pocket money in pence and A stands for our age in years.

Bill is 5 years old. How much does he get?

Amy is 15 years old. How much does she get?

Answer

Bill:

A = 5

P = 5 × 30 + 100 = 150 + 100 = 250 pence

Amy:

A = 15

P = 15 × 30 + 100 = 450 + 100 = 550 pence

Does that make sense?

Let's try some questions on this.