← 20
21
Multiples: ¦¦
42
¦¦¦
63
¦¦¦¦¦
105
¦¦¦¦¦¦¦     
147     
Fractions:   7   3     
Cardinal twenty-one
Ordinal 21st
(twenty-first)
Factorization
Divisors 1, 3, 7, 21
Roman numeral XXI
Binary 101012
Octal 258
Duodecimal 1912
Hexadecimal 1516
Multiples × 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 specific
… of 1:                                
… of 3:                                          
… of 7: