GWD 17 - 2

bobiouou

I can't find any posts on this question. Hope this is not a re-post.

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

Ans = B

My answer is A. Maybe I didn't get through the question. My understanding is: can use SINGLE letter or 2 letters. So only need 4 alphabets. 4 letters + 4*3 > 12

Please help!

chinabens

x + C(2,x)>12 ->x at least is 5

nbnhjf

我觉得是这样吧：关键是对in alphabetic order的理解。

原来我是这样想的，可以这样排,如AB、BA、BC

如果选B的话，肯定不能BA这样排。

如果只用两个字母的话，就是C(X,2)>12,X>=4

既用一个字母又用两个字母，就如一楼所算

X大于等于5即可。