求教几道数学

lizzylovely

170.    18117-!-item-!-187;#058&011424
Of the 13 employees in a certain department, 1 has an annual salary of $38,000, 2 have an annual salary of $45,000 each, 2 have an annual salary of $42,500 each, 3 have an annual salary of $40,000 each, and 5 have an annual salary of $48,500 each.  What is the median annual salary for the 13 employees?

(A) $38,000
(B) $40,000
(C) $42,500
(D) $45,000
(E) $48,500
答案是D, 我是一步步算的。解析里说因为13的中数为7，所以落在45，000的族群里，这怎么一眼看出来啊？

162.    17030-!-item-!-187;#058&010961
If n and y are positive integers and 450y = n3, which of the following must be an integer?
1.    Y/ 3*2^2*5
2.    Y/ 3^2*2*5
3.    Y/ 3*2*5^2
这题是只能用数带进去吗

142.    14580-!-item-!-187;#058&010013
The rate of a certain chemical reaction is directly proportional to the square of the concentration of chemical A present and inversely proportional to the concentration of chemical B present.  If the concentration of chemical B is increased by 100 percent, which of the following is closest to the percent change in the concentration of chemical A required to keep the reaction rate unchanged?

(A) 100% decrease
(B) 50% decrease
(C) 40% decrease
(D) 40% increase
(E) 50% increase
我的理解：A^2与chemical reaction成正比，B与之成反比，B翻了一倍，chemical reaction 变成1/2, 所以A^2就必须也翻一倍。可是比率怎么求呢

冰蓝yiyi

1. 总共13个数，从小到大排列后，中数是中间那个，那就是第七个。从小到大数一下就是45000了。
2. 450=5*3*3*2*5，现在乘个y后变成某数的3次方。意味着y里有2个2，1个3，1个5. 答案是1.
3. 比率是根号2-1

promiselu

142.刚有人教我的  
A2成正比，B成反比，比变成了2倍，所以a变成了根号2倍，根号2-1 约等于0.414， 所以选40%

lizzylovely

恩。谢谢各位