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 ÷ 2 + 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 ÷ 2 + 100 = 150 ÷ 2 + 100 = 75 + 100 = 175 pence

**Amy**

A = 15

P = 15 × 30 ÷ 2 + 100 = 450 ÷ 2 + 100 = 225 + 100 = 325 pence