[求助]后天就考试了，现在还是有不会的题呀~

A certain library assesses fines for overdue books as follows. On the first day that a book 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 4th day it is overdue?

答案是0.7，可我为什么算出来是0.8呢，急需解答，先谢谢大家了~~~

题目是这个意思：从第一天开始算，每天都要用两种方法算过，哪种方法钱少，用哪种。

method1:0.1 0.4(>0.2) 0.5=0.2+0.3(>0.4) 0.7=0.4+0.3

method2:0.1 0.2 0.4 0.8(>0.7)

所以是0.7