1. if 5^21 * 4^11=2*10^n, what is the value of n?
A,11, b, 21, c 22, d,23 e 32
这种题目应该怎么去做呀。。看上去好容易，可是我忘记怎么去approach它了。。好像是有什么规则是不是呢？
把 5^21 * 4^11表示成2*10^n的形式,5^21 * 4^11＝5^21 * 2^22=2*10^21,所以n=21,选b
2. if n and m are positive integers, what is the remainder when 3^(4n+2) is divided by 10?
1. n=2,
2. m=1... answer is B, but why??
这道题有过讨论，我个人认为是有点bug
3. A certain junior class has 1000 students and a certain senior class has 800 students. Among these students, there are 60 siblings pairs each consisting of 1 junior and 1 senior. If 1 student is to be selected at random from each class, what is the probability that the 2 students selected will be a sibling pair?
A, 3/40000, B 1/3600, C. 9/2000, D, 1/60, E, 1/15
(60/1000)*(60/800)=9/2000
4. r s t
on the number line shown, is zero halfway between r and s?
1. s is to the right of zero
2. the distance between t and r is the same as the distance between t and -s.
已知t>s>r,问：s=-r(s>0)?,
1)s>0
2)|t-r|=|t-(-s)|=|t+s|,当s>=0时，s=-r；当s<0时，s=r-2t
所以选选C | 
