Prep math求助大牛

If X and Y are integers, is the value of X(Y+1) even?

(1)X and Y are prime numbers

(2) Y>7

Answer: C both statements together are sufficient, but neither statement alone is sufficient.

问一下为什么不是E？ 即使Y〉7，X，和Y都是质数，也可能X为2，这样就成了偶数，如果不是2 的数也就是奇数咯？这种想法对么。。。

Tanya prepared 4 different letters to send to 4 different addresses. For each letter, she prepared a envelpe with its correct address. If 4 letters are to be put into the 4 envelpe at random. What is the probability that only 1 letter will be put into the envelpe with its correct address? Answer: 1/3

请问怎么做的哦？

第一题，(1)+(2) ==>  Y>7而且Y是质数就意味着，Y一定是奇数，所以Y+1一定是偶数，所以不管X是什么，X(Y+1)都一定是偶数，所以可以确定

第二题 因为只有一封信是到达了正确的地址，所以另外三封信全部都是寄错了。 这种你可以从2个信封开始推，一直推到N个信封，我画了个表格，看一下就知道了。最后是C₄¹ *2/P₄^4 =1/3

当是2个信封

当是3个信封

当是4个信封

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