X is a Roman numeral, a number used before the introduction of Arabic numerals. Roman numerals use seven Roman letters as numbers, i. e. I (1), X (10), C (100), M (1000), V (5), L (50), D (500). Roman numerals originated in ancient Rome. There are seven numeric symbols: I, V, X, L, C, D, M. When using Roman numerals to represent numbers, if several identical numbers are juxtated, it means that the value of the number is several times that of the number. For example: Roman numerals to represent 3, can be written as Ⅲ; To represent 20, we could write XX; To represent 30, write XXX. When different digits are juxtaposed, if the smaller digit is on the right, it means that the value of the number is the sum of these digits; If the smaller digit is on the left, it means that the value of the number is the difference between the digits.
Answer: X is the number ten.
Analysis: In addition to 0,1,2,3, 4,5,6,7,8,9 on the ID card, the 10 numbers are represented by X.
Discussion: X stands for ten,
X| is for eleven
X|| means twelve,
|X means nine, etc., will not be listed one by one, this is a new way to express numbers, which is often used in higher mathematics, and is often used in production and life, for reference only.