<求助>請教PP3幾道數學

題目有點看不懂 請教思路

1)A certain restaurant offer 6 kinds of cheese and 2 kinds of fruit for its dessert platter, if each platter contains an equal fo kinds of cheese and kinds of fruit, how many different platters could the restaurant offer?     ANS:27

2)In a class of 30 students, 2 students did not borrowed any bookds from the library, 12 students each borrowed 1 book, 10 of the students each borrowed 2 books, and the rest of the students each borrowed at least 3 books. If the average number of the books borrowed per student was 2, what is the maximum number that any single student could have borrowed?     ANS:13

3)Store S sold a total of 90 copies of certain book during the seven days of last week, and it sold different numbers of copies any two of the days. If for the seven days the store S sold the greatest number of copies on Saturday adn the second greatest number of copies on Friday, 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

ANS:B

(1)each platter contains an equal fo kinds of cheese and kinds of fruit是说可以是1种cheese+1种fruit,也可以是2种cheese+2种fruit,把两种情形分别算出来，相加就可以了。                          C(6,1)*C(2,1)+C(6,2)*C(2,2)

(2)没看懂。

(3)第一个无法判断，Friday取10和12都满足要求。第二个比较tricky，判断方法如下：

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因而x在最小的情形下都大于11，其他情形肯定大于11。

(2)  # of students vs # of books: 2--0, 12--1, 10--2, 6 equal or greater than 3.

Only when 5 of the 6 students borrow 3 books, we can have one single student borrow the maximum number of books to get the average number, which is 2. 

So, we have (2*0+12*1+10*2+5*3+x)/30=2  x=60-47=13