Roman Numerals / 21

21 in Roman Numerals

XXI

21 = XX (10), I (1). That's XXI, purely additive. 21 splits into 2 tens (XX), 1 (I), together forming XXI. One below 21 is XX (20); one above is XXII (22). XXI runs 3 symbols with 2 value-chunks, reaching 21. 21: 2 Arabic digits, 3 Roman symbols, for 21. In binary, 21 is 10101, 5 bits long for 21.