提问几道gre数学题都是绿皮上的1.A certain interger n is a multiple of both

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提问几道gre数学题
都是绿皮上的
1.A certain interger n is a multiple of both 5 and 9.which of the following must be true?
A.n is an odd integer B.n is equal to 45 C.n is a multiple of 15
A选项为啥不对?
2.How many positive integers less than 20 are equal to the sum of a positive multiple of 3 and a positive multiple of
答案是seven.这题怎做啊,除了一个数一个数代还有啥方法?
3.A buzzer sounds every 15 minutes.if the buzzer sounded at 12:40,which of the following could be a time at which the buzzer sounded?
A.4:05 B.5:30 C6:45 D.7:15 E:8:10
4.If n is an odd integer,then n^2-1 must be
A.a prime number B.an odd integer C.divisible by 8 D.a multiple of 2^n E.a positive integer
就想问D如何排除?

这个数字是5和9的倍数,也就是说,一定是45的倍数,但不一定是奇数,比如90就满足条件.但是,只要是45的倍数,就一定是15的倍数.
3的倍数和4的倍数的和,还要小于20,只能逐个尝试,我试出来8个.
7=3+4
10=3*2+4
11=3+4*2
13=3*3+4
14=3*2+4*2
16=3*4+4
17=3*3+4*2
19=3*5+4
这个题恐怕要逐个验证一下,就是看看那段时间是否会响.
n是奇数,写作2m+1,则n^2-1=4m(m+1)
m和m+1是连续数,乘积必为偶数,所以选C.如果想去掉D,就找个例外,比如n=5,n^2-1=24,就否掉了.
再问： divisible by 8是什么意思，这个数必须比8大吗？若n^2-1是个小于8的整数，那还叫divisible by 8 吗
再答： divisible by 8 意思是能被8整除。题目说n是奇数，当n=1时，0能被8整除；当n=3时，8能被8整除，n再大一些，就都大于8了。

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