问一道最新PREP数学题

mingmings

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 results in the lesser amount. What is the total fine for a book on the fourth day it is overdue?
A. 0.60
B 0.70
C 0.80
D 0.90
E 1.00
答案是B，请大神赐教

saffsa761

感觉上是这样：
第一天罚金是0.1，接下来每一天罚金增加的规则是要么增加0.3要么是其两倍，并且是取二者之间小的那一个
所以：
第一天 0.1
第二天 0.4或者0.2，二者取小，则0.2
第三天0.5或者0.4，同理，取0.4
第四天0.7或者0.8，二者取小，则0.7

所以答案是0.7

mingmings

多谢指点，明白了！！