请教几道GMATPrep的math

According to the derections on a can of frozen organge juice concentrate, 1 can of concentrate is to be mixed with 3 cans of water to make organge juice. How may 12-ounce cans of the concentrate required to prepare 200 6-ounces serving of orgage juice

25    34    50   67   1000

ans:25

题目不懂

susan drove at an average speed of 30 miles per kilemeter for the first 30 miles of a trop and then at an average speed of 60 m/k for the remaining 30 miles of the trip. what was S's average speed entire trip?

25   40    45   50   55

mine:45   ans:40

why?

a certain library assesses fines for overdue books at follows. One the fisrt day that a book is overdue, the total fine is 0.1$. For each additional day the book is overdue, the total fine is either increased by 0.3$ or doubled, whichever results in the lesser amount. What is the total fine for a book on the fourth day it is overdue.

0.6      0.7       0.8     0.9    1

mine:0.9   ans:0.7

why?

concentrate : water = 1:3, 问需要多少cans of 12-ounce的concentrate来对200cans of 6-ounce的orange juice.
(200*6*1/4)/12=25

speed=s/t=(30miles+30miles)       

               [(30miles/30)+(30miles/60]

真是糊涂，我居然用了求平均数的公式

我的答案是0.8雅,

超期的天数:                          1         2         3        4

方法一double)                 0.1      0.2      0.4      0.8

方法二0.1+(n-1)*0.3)       0.1      0.4      0.7      1.0

有谁可以帮忙看看嘛?

1:3

1+3=4

12X*4=200*6

X=25