When we substitute a number for a letter, we simply replace the letter with the number.

However, we must remember that 4b means 4 x b.

Example

Work out the value of the following expression when a = 4

3a - 7

Answer

3a - 7

= 3 x a - 7        (3a just means 3 x a

= 3 x 4 - 7        (Change the letter into the number you are given)

= 12 - 7            (Remember BODMAS; Multiply first)

= 5