**Substitution** *KS3 Revision*

## What you need to know

**Things to remember:**

- Substitution just means replacing a letter with a number.
- We don’t write \times when there is a number before a letter, so you need to remember that it is hiding there!
- A fraction is another way of writing a division question.

So, what is substitution? Well, it just means to “replace” one thing with another. So, in maths, this usually means replacing a letter with a number.

Use substitution to find the value of x+7 when x = 12

Here we are told that x=12, so all we have to do is replace the x in the expression with 12!

x+7=12+7=19

Easy peasy! We do the same with our other operations too!

Use substitution to find the value of x-4 when x = 15

x-4=15-4=11

Multiplication questions are a little different, because we need to remember that there is a hidden \times symbol between the number and letter.

Use substitution to find the value of 5x when x = 13

5x=5\times x=5\times13=65

Divisions questions could either appear as a division:

Use substitution to find the value of x\div3 when x=9

x\div3=9\div3=3

Or as a fraction that we have to change:

Use substitution to find the value of \frac{20}{x} when x=5

\frac{20}{x}=20\div x=20\div5=4

## Example Questions

**Question 1:** Use substitution to find the value up 12x when x=7

12x=12\times x=12\times7=84

**Question 2:** Use substitution to find the value up \dfrac{x}{9} when x=54

\dfrac{x}{9}=x\div9=54\div9=6