22.（DS）一个含有10个数字(1,3,5,7,9,11,13,15,17,19)的数列的标准方差问题。问方差是否不变？
1)抽掉2个数后，mean不变；
2)抽掉2个数后，sum为90
选E
1)可抽任意两个相对mean相等的数
2)可抽任意两个和等于10的数 eg.1,9 or 3,7
should be D, 因为方差总是会变的。
216. 问95年到97年的享受保险的员工percentage增加了多少？
a. 96年比95年增加了多少percentage
b. 97年比96年增加了多少percentage
[确认]E
should be C: 96=95*(1+x%), 97=96*(1+y%)=95(1+x%)(1+Y%), since x,Y are known, the key is there.
281, from 0, 1, 2, 3, 4, 5, 6, 7,8, 9 chose 3 number to form a digit. the first number of the digit can not has 0, 1, the second number can only use 0 or 1. if the second number is 0, the third number can not be 0. How many possible ways?
旧JJ：A three digit code, composed of 0-9 ,其中第一位不能为0 和1，第二位只能为0或1，且第二、三位不能同时为问有多少种不同的Code
[思路]8x2x9=144
my key is 152= C(1 8)C(1 9) +C(1 8)C(1 10)
366.A mixture is composed of 5 gallons of water and 5 gallons of antifreezer. Now use 3 gallons of the mixture and add 3 gallons of antifreezer to the mixture. What is the percentage of antifreezer in the mixture now?
[确认] 65%
