The characters for the number twenty in the United States are represented as "20." This numeral consists of the digit "2" followed by the digit "0." The "2" signifies the tens place, while the "0" indicates that there are no units. The use of the numeral "20" is standardized in English-speaking countries, including the United States, and is widely recognized across various contexts such as mathematics, finance, and everyday communication.

The reason for this representation lies in the decimal numeral system, which is the most widely used numeral system in the world. In the decimal system, each digit's value is determined by its position within the number. The first digit on the left represents the ones place, the second digit represents the tens place, the third digit represents the hundreds place, and so on. This positional notation allows for a concise and efficient way to represent large and small numbers.

According to the "Number Sense: Rethinking Mathematics for Elementary School Teachers" by John Mighton, the decimal numeral system is a result of historical developments and the need for a system that is both practical and efficient. The use of "20" as the representation for twenty is a direct application of this system, ensuring consistency and clarity in numerical communication.

In the United States, the numeral "20" is taught in early education and is reinforced through various educational resources and materials. For instance, the "National Council of Teachers of Mathematics" (NCTM) emphasizes the importance of understanding place value and the decimal system in mathematics education. Data from the NCTM's "Principles and Standards for School Mathematics" indicate that understanding the decimal system is crucial for students to develop a strong foundation in mathematics.

Furthermore, the numeral "20" is consistent with international standards. The International Organization for Standardization (ISO) recognizes the decimal numeral system as the international standard for numerical representation. This standardization ensures that the numeral "20" is universally understood, facilitating global communication and collaboration.

In conclusion, the characters for the number twenty in the United States are represented as "20," which is a direct application of the decimal numeral system. This system is widely used due to its practicality and efficiency, and its representation of "20" is consistent with international standards and educational practices.