求助：几道og模考软件里的数学题

sharro111

想了很久，都没有搞清楚，烦请各位多多指点。。。。

1，what is the sum of the different positive prime factors(不同的正因数）of  550? A,10   B,11  C,15  D,16  E,18

2,the residents of town x participated in a survey to determine the number of hours per week each resident spent watching television(每人每周看电视的hours), the distribution of the results of the survey had a mean of 21 hours(平均数为21） and a standard deviation of 6 hours（标准差为6）.the number of hours that Pat, a resident of town x(Pat 是一个居民），watched television last week was between 1 and 2 standard deviation below the mean , which of the following could be the number of hours that Pat watched television last week?  A,31  B,20  C,18  D,12  E,6

3,If x is an integer(整数）， is (x^2+1)(x+5) an even number(偶数）？（1）x is an odd number(奇数） （2）each prime factor of x^2 is greater than 7        A,直到条件1即可解出此题  B,知道条件2即可  C,两条件合起来才能解  
D,两条件分别都可解出   E，两个条件都不够充分，无法回答以上问题

另外还有一个概率上的概念P(A 交B )=P(A)XP(B )此公式并非必然成立，绝对成立的公式:  P(A 交B )=P(A)XP(B/ A)，其中P(B/ A)表示A发生的情况下B发生的概率，当A和B相互独立的时候P(B/ A)=P(B )，那么，P(B/ A)的公式是怎样的呢？ 

sharro111

请问一下Between 1 and 2 standard deviation below the mean
是什么意思呀？？