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- Vigesimal

The **vigesimal** or **base-** numeral system is based on twenty (in the same way in which the ordinary decimal numeral system is based on ten).

In a vigesimal place system, twenty individual numerals (or digit symbols) are used, ten more than in the usual decimal system. One modern method of finding the extra needed symbols is to write ten as the letter A_{20} (the _{20} means base 20), to write nineteen as J_{20}, and the numbers between with the corresponding letters of the alphabet. This is similar to the common computer-science practice of writing hexadecimal numerals over 9 with the letters "A-F". Another method skips over the letter "I", in order to avoid confusion between I_{20} as eighteen and 1 (one), so that the number eighteen is written as J_{20}, and nineteen is written as K_{20}. The number twenty is written as 10_{20}.

According to this notation:

20_{20} means forty in decimal