How did you do it without asking your last name?

Actually, this is a math problem. I always told us in high school that you can ask your math teacher.

There are hundreds of surnames commonly used by China, so he numbers them in turn.

Then his last name is expressed in binary, if Li is 4.

This binary number is 100, so he wrote Li on the paper marked with number 3.

However, because the binary system is too simple and easy to be found, fortune tellers usually take a larger number, such as 2 1 that is 1 kloc-0/01,then he will write the surname on the card1,3,5.

Of course, he will confuse the cards, and you can't tell which one it is, but the fortune teller knows, so that he can get your last name by binary algorithm as long as he asks if there is your last name in the card.