Mathematics for IGCSE & O level - Using A Calculator (Section 2 - No. 3)
What are the 3 options to help check the answer?
Use some method of estimating.
Make sure you've followed the order of operations.
Use a completely different method to calculate the same answer.
Make sure the calculator is working correctly.
Explication
Estimating the answer and following the correct order of operations, will help to ensure the correct answer.