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Mathemagic: Find the trick!

Which of the following methods cannot be used as a trick by a mathemagician to find your final answer?

A. Think of a single-digit positive integer. Multiply by 37 and add 74. Multiply by 3 and then subtract 222. Divide by your initial number. I can tell you what the answer is!

B. Think of a positive integer. Multiply by 10. Add 9 and subtract the original number. Now find the repeated sum of digits. I can tell you what the answer is!

C. Think of a positive integer. Multiply the number by 10. Find the repeated sum of digits. Subtract this from your original number. I can tell you what the answer is!

Note: Repeated sum of digits means that you take the digit sum of the number repeatedly until you end up with a single-digit number.

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