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 = 0.4
3a - 1
Answer
3a - 1
= 3 x a - 1
= 3 x 0.4 - 1
= 1.2 - 1
= 0.2