When we substitute something, we replace one thing with another (like in football, one player comes off and another one goes on).

In maths, it means to replace a letter (variable) with a given number.

When a = 3 we can work out the value of the expression:

4a - 7

Remember 4a means 4 x a

Answer

4 x 3 - 7

a has been replaced by 3

Which gives us 4 x 3 = 12

12 - 7 = 5

Did you spot the use of BIDMAS here? We need to multiply before we subtract.

Have you got this?

We can do this with lots of substitutions.

Example

When a = 6 and b = 8

What value is the expression a + b?

Answer

Let's substitute a with 6 and b with 8

a + b = 6 + 8 = 14      Easy!

Are you ready to have a go at some questions?