问几道关于统计的题目（英文），希望大神解答！

问几道关于统计的题目（英文），希望大神解答！
初学者，题目应该不难。。但是我不太理解。。
1、A political scientist wants to know how college students feel about the social security system. She obtains a list of the 3456 students at her college and mails a questionnaire to 250 students selected at random. Only 104 questionnaires are returned. What was the sample size?
a. 104
b. 250
c. 3456
请问sample size怎么求？
2、（1）A sample of 100 Stat 1060 midterm scores showed a bell-shaped distribution with a mean of 70 and standard deviation of 10.
What is the percentage of students who get scores less than 50?
a.16%
b.5%
c.2.5%
d.0.3%

（2）What is the percentage of students who get scores between 50 and 90?
a.68%
b.99.7%
c.95%
d.2.5%

（3）What is the minimum score if a student gets it, will make him/her among the top 2.5% .
a.70
b.80
c.90
d.100

（4）What is the percentage of students who get scores between 60 and 90?
a.95%
b.68%
c.81.5%
d.There is no enough information.
希望大神讲一下具体算法，还有我知道在1standard deviation 是68%，2SD是95%，3SD是99.7%，但是我不知道这到底咋看是怎么运用在bellshape的？希望大神指点