在自然數1~20中,質數有
2,3,5,7,11,13,17,19
2,3,5,7,11,13,17,19
,合數有
4,6,8,9,10,12,14,15,16,18,20
4,6,8,9,10,12,14,15,16,18,20
;奇數有
1,3,5,7,9,11,13,15,17,19
1,3,5,7,9,11,13,15,17,19
;偶數有
2,4,6,8,10,12,14,16,18,20
2,4,6,8,10,12,14,16,18,20
,既是奇數又是合數的數有
9、15
9、15
,既是偶數又是質數的數有
2
2
,既是奇數又是質數的數有
3,5,7,11,13,17,19
3,5,7,11,13,17,19
,既是偶數又是合數的數有
4,6,8,10,12,14,16,18,20
4,6,8,10,12,14,16,18,20
.