求解prep数学！

leechole

A certain library assess fines for overdue books as follow on the frist day that a book is overdue The total fine is 0.1$.For each addtional day that a book is overdue the total fine is either increased by 0.3 or doubled whichever result in the lesser amount that is the total fine for a book on forth day it is overdue ?
A.0.6     B.0.7      C.0.8      D.0.9      E.1.0
表示没搞清楚他的意思 答案是B来着。。。。。。

rainyyla

就是在double 和+0.3的结果中取消的，第一天0.1，第二天double 为0.2，第三天double为0.4，第四天加0.3，结果是0.7

leechole

诶哟我去啊 可以合起来用的哇。。。