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?
這一題我不知道怎樣算才快, 好像沒好法子
(1) m is not divisible by 3.
根據題目 : 既不能是偶數又不能被三除
5,7,11,13,17,19,23,29 <== 是因為不管m是哪一個數, 都只有一個prime number可以除他嗎? sufficient (不是很懂題目要考什麼思路?)
(2) m is not divisible by 5.
3,7,9,11,13,17,19,21,23,27,29 <== 其中21可以有兩個prime numbers, 因此insufficient
Answer : A