求助：Gwd 第一套 数学Q2,4,5,13

明教之徒

Q2.A researcher plans to identify each participant in a certain medical experiment with a code consisting of either a single letter or a pair of distinct letters written in alphabetic order.What is the least number of letters that can be used if there are 12 participants,and each participant is to receive a different code?
A.4 B.5 C.6 D.7 E.8
各位大侠，此题的解题思路是什么啊？

明教之徒

答案为B.5

明教之徒

Q13。Juan bought some paper books that cost $8 each and some hardcover books that cost $25 each.If Juan bought more that 10 paperback 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.

答案选C
我选E了，因为即使把两个条件都放在一起，只能求出paperback的数量是6 或7，从而hardback的数量也不确定啊。
at least 是大于等于的意思吧，greater than 是大于的意思吧。

明教之徒

大家别不睬我啊，