this before using a different method but it works the same way every time. The key is to add the numbers you write down. All of the answers of the subtaction are divisible by 9. So when you type in your answer minus the one you circle, their is a lookup table containing all of the answers possible for numbers divisible by 9. It simply matches digits in all combinations with correct answers and selects the one match supplying your circled digit. Anyway that's how I did it.