问一道PREP题

chinabens

Store S sold a toat of 90 copies of a certain book during the seven days of last week, and it sold different numbers of copies on any two of the days. If for the seven days Store S sold the greatest number of copies on Saturday and the second greatest number of the copies on Friady, did Store S sell more than 11 copies on Friday?

1. Last week store S sold 8 copies of the book on Thursday.

2. Last week store S sold 38 copies of the book on Saturday.

Answer: B

请问高手，如何快速地在2分钟内解出这题？我脑子有点膨胀。

chinabens

thanks !!!!![em08]

uniqueness

设FRIDAY是X,因为每天都不同，使得X最小的情形是：38+x+x-1+x-2+x-3+x-4+x-5=90

6x=67

x=67/6>11