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？

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??

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

我的排列组合不是很好，有没有哪位可以教教我怎么做呀？？

在问一道题目

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.

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

这道题有过讨论，我个人认为是有点bug

(60/1000)*(60/800)=9/2000

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

答案是A,

哦，对，是我搞错了，忽略了在一个年级里选了一个学生之后，另一个年级别无选择，只有一个人才是先选的这个人的sibling pair。

所以应该是(60/1000)*(1/800)=3*40000