Prep -1 请教2道数学与1道语法题

tracyforever

数学：

A certain junior class has 1000 students and a certain senior class has 800 students. Among these students, there are 60 sibling pairs, each consisting of 1 junior and 1 senior. If 2 students to be selected at random form 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

Maybe i think it too complicated, anyway i can't calculate an answer here.


At least 100 students at a certain high school study Japanese. If 4% of the students at the school who study French also study Japanese, do more students at school study French than Japanese?
1) 16 students at the school study both French and Japanese
2) 10% of the students at the school who study Japanese also study French

I chose both 1) and 2), can't understand who 1) can't fullfil here... i think the meaning shall be the same as 2)

语法：

Concerns about public health led to the construction between 1876 and 1904 of three seperate sewer systems to serve metropolotan Boston.

B) Concerns about public health have led to the construction of three seperate sewer system between 1876 and 1904 to serve...

here i can't understand why "led" instead of "have led" is used here... why "have led" is not acceptable?

Appreciate NN's reply and advice. Thanks alot!!!

tracyforever

谢谢！

sissizhx

现在完成时 表示动作发生在过去，但与现在的情况有联系 表示从过去某一时间开始一直延续到现在并还可能继续延续下去的动作，在此 between 1876 and 1904清楚地说明动作已经结束~~所以用过去式。

数学1：由于每个学生根他的的sibling是一一对应的，i.e. 抽到A学生的sibling一定是A’，则同时抽到A和A'的概率是1/1000* 1/800，而又有60对这样的sibling，所以，概率为 60 * 1/1000 * 1/800

再说一下数学2：1）中说16个人同时学习日语和法语。题目的前提说占4%， 也就是说总共有 16/4%=400个人的总学生人数，他说100+的学生学习日语，那可能有200个人学习法语，还有100个什么都不学。所以不能证明学习法语的人一定比学习日语的要多。

不知道我说明白没有啊，

tracyforever

自己顶起来！