Researcher -Coding

vinay1983 wrote: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 alphabetical 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

OA: 5

Since the answer choices are so small, we might consider testing cases.
So, at this point, we need to figure out where to begin. This is where we need a bit of number sense.
I chose to start with A, because I could think of several possible codes using 4 letters.

So, let's manually list all of the codes that can be made with 4 letters
We get: A, B, C, D, AB, AC, AD, BC, BD, CD
10 possible codes in total (we need at least 12 codes)

So, we need to use MORE LETTERS.
At this point, it should be obvious that adding 1 letter will give us at least 2 extra codes, so the answer is [spoiler]5 letters (B)[/spoiler]

Cheers,
Brent