1道搞不懂的Prep数学题

A certain library assesses fines for overdue books as follows. On the first that a book is overdue, the total fine is 0.1 dollar. For each additional day that the book is overdue, the total fine is either increased by 0.3 dollar or doubled, whichever  results in the lesser amount.  What is total fine for a book on the forth day it is overdue?

A. 0.6 dollar  B 0.7 dollar  C 0.8 dollar  D 0.9 dollar  E 1.0 dollar

答案是B, 我算了是C。下面是我的理解，大家看看错在哪里？

         罚金+0.3       罚金*2

day1  0.1

day2  0.4                0.2

day3  0.7                0.4

day4  1.0                0.8            

这样一来，第四天最低应该是0.8元啊！错在哪里？

你好

total fine is either increased by 0.3 dollar or doubled

也就是说加0.3或者double都是在今天的实际fine的基础上，表格应该为

加0.3                  double            实际

day1  0.1                                       0.1      第一天

day2  0.1+0.3            0.2               0.2      取double

day3  0.2+0.3            0.4               0.4      取double

day4  0.4+0.3            0.8               0.7      取+0.3

不知道这样理解对不对，这么计算答案应该是0.7

严重同意！