奇数有:1,3,5,7,9,11,13,15,17,19 偶数有:2,4,6,8,10,12,14,16,18,20 质数有:2,3,5,7,11,13,17,19 合数有:6,8,9,10,12,14,15,16,18,20
奇数:1 3 5 7 9 11 13 15 17 19,偶数:2 4 6 8 10 12 14 16 18 20质数:2 3 5 7 11 13 17 19合数:4 6 8 9 12 14 15 16 18 20