The word substitute means to replace. When a question asks you to make a substitution, it is asking you to replace all the letters with their corresponding numbers. There are only three steps you need to remember:
Step 1: write down the FORMULA which will be given as part of the question
Step 2: INPUT all the numbers you have been given into the formula
Step 3: Use BODMAS etc to SOLVE the sum and get the answer
You can easily remember these three steps as ‘Taking the FIS’ – be careful how you say it!!!!
the first letters of Formula
Input
Solve
spell the word FIS.
Eg.