1. Think of your age (or any number greater than 9).

2. Multiply by 10.

3. Now pick a number less than or equal to 9.

4. Multiply it by 9.

5. Subtract the two results (step 2 and step 4).

6. Ask for the answer.

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To determine the age (or the number picked),
take the units digit of the answer and add it to the rest of the number.



For example:

1. Think of your age (or any number greater than 9).   Let's think of 136.
2. Multiply by 10.   Then we get 1360.
3. Now pick a number less than or equal to 9.   Let's pick 3.
4. Multiply it by 9.   Now we get 27.
5. Subtract the two results (step 2 and step 4).   We get 1360 - 27 = 1333.
6. Ask for the answer.   We are told 1333, so add the units digit (3)
  to the rest of the number (133)
  to obtain 136 (the original number).