Grab a pencil. Ask someone to choose a number with at least 3 digits but not more than 8.
Ask them to add up the digits.
Subtract the sum from the first number.
Have them select one digit from the new number and strike it out.
Add the remaining digits.
Ask them to tell you the last number.
You can tell them what digit they struck out. How? Subtract the number they give you from the next multiple of 9. In our example, the next multiple of 9 is 1×9=9. 9-7=2.
Try different numbers.
Is there ever a time that this doesn’t work?
Can you figure out how the trick works?