2.將1~500的正整數依序刪2.3.5的倍數後變做的數依序由大到小請問257排第幾個?
[2,3,5]=30
1,7,11,13,17,19,23,29
30-[30/2]-[30/3]-[30/5]+[30/6]+[30/10]+[30/15]-[30/30]
=30-15-10-6+5+3+2-1
=8
257/30=8….17
8*8+5=69
2.將1~500的正整數依序刪2.3.5的倍數後變做的數依序由大到小請問257排第幾個?
[2,3,5]=30
1,7,11,13,17,19,23,29
30-[30/2]-[30/3]-[30/5]+[30/6]+[30/10]+[30/15]-[30/30]
=30-15-10-6+5+3+2-1
=8
257/30=8….17
8*8+5=69