183. 17831-!-item-!-187;#058&011633
If m is a positive odd integer between 2 and 30, then m is divisible by how many different positive prime numbers?
(1) m is not divisible by 3.
(2) m is not divisible by 5.
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【答案】A
【思路】
2<m<30
{3,5,7,9,11,13,15,17,19,21,23,25,27,29}
(1) m excludes {3,6,9,12,15,18,21,24,27}={5,7,11,13,17,19,23,25,29} 全部只可以被自己和1除
我有疑問的是25不是也可以被5除嗎??
(2) m excludes {5,10,15,20,25}={3,7,9,11,13,17,19,21,23,27,29} 21除了自己和1還可以被7除
答案為什麼不是C呢?
共同的質數(7.11.13.17.19.23.29)
詳解我看不太懂希望有人可以幫忙
謝謝!