一道PREP数学模考题

A certain library assesses fines for due books as follows. On the first day that boook is overdue, the total fine is $0.1. For each additional day that 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?

Answer: $0.7.

我选的是$0.8. 我的思路是：

if increased by $0.3, on the fourth day: 0.1+0.3+0.3+0.3=1

if doubled:  0.1*2*2*2=0.8

取值小的那个，那么是$0.8

为什么prep给的答案是0.7呢？请大家帮忙解答一下，谢谢。

第一天,0.1

第二天,0.2 因为小于 0.1+0.3

第三天,0.4 因为小于 0.2+0.3

第四天,0.7 因为小于 0.4*2