Prep math求助大牛

weiqite

1)In the class of 30 students, 2 students did not borrow any books from the library, 12 students each borrowed 1 book, 10 students each borrowed 2 books, and the rest of the students each borrowed at least 3 books. if the average(arithmetic mean) number of books borrowed per students was 2, what is the maximun number of books that any single student could have borrowed?

3,5, 8, 13, 15

Answer:13

Mine:8

arithmetic mean=2, then 28*2=56 books in all. 56-12-10*2=24(the rest of students book), number of students=28-10-12=6, at least 3, so the maximun should be 8. why 13?

2) Juan bought some paperbooks that cost $8 each and some harcover books that cost $25 each. If Juan bought more than 10 paperpack books, how many hardcover books did he buy?

(1)the total cost of the hardcover books that Juan bought was at least $150

(2)The total cost of all the books that Juan bought was less than $260

answer:C

Mine:E

解出来的答案是2.8<X<7.2所以就选E了，为什么不对哦？

谢谢

maosharon

1)  arithmetic mean=2, then 30*2=60 not 28*2=56 books in all. 60-12-10*2=28(the rest of students book), the rest number of students=28-10-12=6, at least 3 for each rest student,then 28-5*3=13, so the maximun should be 13.

2) in the term of both (1) and (2), only one answer can be conducted.

----suppose that the cost of hardcover is more than $150, then the quantities of hardcover books are >=7, and the cost is >=$175, then the cost for paperbooks is

<=$85(260-175=$85). conflict the the given term: Juan bought more than 10 paperpack books(which will cost at least $88).

-----The only answer is that Juan bought 6 hardcover book.