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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
