Vigesimal Explained

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).

Places

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 A20 (the 20 means base 20), to write nineteen as J20, 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 I20 as eighteen and 1 (one), so that the number eighteen is written as J20, and nineteen is written as K20. The number twenty is written as 1020.

According to this notation:

2020 means forty in decimal