[求助] 算了半天也很有问题的PREP数学啊？

我做PREP的数学遇到两道题目，非常奇怪，算了好久也是和答案不怎么一样。不知道是不是理解题目有问题，希望NN们指点啊！

A certain library assesses fines for overdue books as follows. On the first day that a book is overdue, the total fine is $0.10. For each additional day that the book is overdue, the total fine is either increased by $0.30 or doubled, whichever in the lesses amount. What is the total fine for a book on the fourth day it is overdue?

A. 0.6   B 0.7  C 0.8  D. 0.9  E. 1.0

答案是B，可是我算出来是C。=（

Each employee on a certain task force is either a manager or a director. What percent of the employees on the task force are directors?

1. the average salary of managers on the task force is $5000 less than the average salary of all employees on the task force

2. the average salary of directors on the task force is $15000 more than the average salary of all employees on the task force

答案是C，可是我觉得是E。=（

希望NN帮忙啊～

第二题

设the average salary of all employees =p,managers=m,directors=d

(m+d)P=m(p-5000)+d(p+15000）

得出3d=m

第一题树状图做比较清楚~  LZ可能没有读懂题干吧? 

2种计算罚金的方法按少的一种计,算出来是B没错~

first day:0.1

2nd: 0.2

3rd:0.4

4th:0.4+0.3=0.7

第二题：C

设manager人数为m,平均工资为x;director人数为d，平均工资为y，总平均工资为z。

则由总工资＝manager工资＋director工资可得：z（m+d)=mx+dy

又因为x=z-5000,y=z+15000,可得z(m+d)=m(z-5000)+d(z+15000)

化简后就是d/m=1/3,所以可以求出director在总人数中占了25%。

非常感谢大家的帮助，突然明白了第一题我的理解有错误了，我以为一种是一直+0.3 加加加上去，然后一种是一直DOUBLE DOUBLE， 原来两种是要同时考虑的。

第二题由于大家的清楚的解释，我也恍然大悟了。真是非常感谢大家。

希望以后我也能帮助各位！ =）