 Substitution Algebra Revision | KS3 Maths Resources

## 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

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

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

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