As part of simple card trick, I want to allow a user to choose a number between 1 and 100 and then ask them to do various maths to lead them to the same number so their choice becomes irrelevant.
One way to do this is ask them to multiply their chosen number by 9. Then ask them to repeatedly sum each individual digit until they get to a single digit number. This will always come back to 9 and thus you have a base to then transition to the desired number.
Are there any other ways to do this (ideally something a bit more subtle)