求解释，数学prep里面的一道原题

赵菲比

Last month a phone company charged $0.07 per minute for the first 300 minutes of calling time and $0.05 per minute for each minute of calling time in excess of the first 300 minutes. For how many minutes of calling time was Bob charged by the phone company last month?

(1) Last month the phone company charged Bob $21.00 for his calling time that was charged at the rate of $0.07 per minute.
(2) Last month the phone company charged Bob $40.00 for his calling time that was charged at the rate of $0.05 per minute.


不是很理解，为什么答案是B
我理解的题目的意思是说头300分钟o.o7美元。后面超过300的每一分钟是0.05美元。
$0.05 per minute for each minute of calling time in excess of the first 300 minutes 这句话不是这个意思吗？

赵菲比

而且（1）都已经说明了是按照0.07来收费的，为什么还会是unknown呢？

Jasper_Zhou

(1)中“ for his calling time that was charged at the rate of $0.07 per minute.”的意思是，0.07部分，打了21块钱。
可是实际上0.07部分，打满300分钟也是21块钱，所以只是知道0.7部分打了300分钟，不知道后面的。
(2)中，0.05部分打了40块，也就是打了800分钟，因此可以求得加起来打了300+800=1100

vonchiang

Jasper_Zhou 发表于 2020-1-7 17:06
(1)中“ for his calling time that was charged at the rate of $0.07 per minute.”的意思是，0.07部分， ...

同意！               

Kiron

感谢