[求助]prep里面的几道题，请帮助,多谢！

when t is a positive integer and r is the remainder when t^2+5t+6 is divided by 7, what is the value of r?

1. when t is divided by 7, the remainder is 6

2. when t^2 is divided by 7, the remainder is 1

答案是 a

Is the integer n odd?

1. n is divisible by 3

2 2n is divisible by twice as many positive integers as n

答案是 b

Tanya prepared 4 different leeters to be sent to 4 different address, for each letter, she prepared an envelope with the correct address. If each of the 4 letters are to be put in one the four envelopes randomly, what is the probability that only one letter will be put in the correct envelope?

答案是 1/3

请问有人可以提供以下解题思路么？谢谢了

还有一个问题， point x,y lie in the same plane as circle o,这里这个same plane 是什么意思啊？仅仅是指同个平面么？还是他们之间是有重合的？

第2题

（1）明显不对

（2）题干的意思是2n的因子数是n 因子数的2倍，如果n 是偶数，那么2是n的因子，而2n的因子只比n的因子多一个，即多出2n这个因子本身。而如果n为奇数，2不是n的因子，而显然2是2n的因子，而“2*n的因子”一定是2n 的因子一定是2n的因子。所以2n的因子数是n因子数的两倍。

第3题

假设1、2、3、4号信，1、2、3、4号address。数字对应为correct

（1）先从4封信中选一封寄对的（C41）。比如选中了1号信寄到1address

(2)再考虑2号信，此时2号信只有两种选择，即3、4 address  C21

(3)3号信显然只能寄到4address ,4号信只能寄到3address.也就是说这两封信没有选择

所以  C41*C21/A44

谢谢阿

但是关于信分那道还是不太懂。。。