Cinematiccuisine wrote:
Amy is organizing her bookshelves and finds that she has 10 different types of books. She then codes each book with either a single letter or a pair of two different letters. If each type of book is uniquely represented by either a single letter or pair of letters, what is the smallest number of letters Amy will need to create the codes for all 10 types of books? (Assume the order of letters in a pair does not matter.)
A) 3
B) 4
C) 5
F) 10
E) 20
Questions:
1) How do i rephrase this question? The Kaplan answer is not so easy to understand ( at least for me)
2) without getting into too many calculations, how can i solve this?
Thank you for your help.
Hi
Cinematiccuisine!
Happy to help
Since the numbers we're dealing with here are small, it is probably easiest to just solve this manually, at first, at least to understand what the question is asking about. Amy is labeling each type of book with either one or two letters. If she is using just one letter, this means there is only one code she can use:
A
If she is using two letters, then there are three:
A
B
AB
We are told that the order of the letters doesn't matter, so we don't have to worry about counting AB and BA separately. Now, with three letters, we have all of the above combinations, plus three more:
C
AC
BC
That's six total. If we add one more, then we have those six plus four more:
D
AD
BD
CD
And that give us 10 possible codes, which is what the question is asking for
So the answer is 4.
Now, if the number was much bigger, then we might need to create a formula to solve this. The easiest way to do that is to just notice the pattern. Every time we add a new letter, we get one more code (just that letter), plus the number of letters that we already have, which create the combinations of mixed letters (like AD, BD, CD). So adding C gave us 3 more codes, adding D gives us 4 more codes, adding E gives us 5 more codes, and so on. We can use that pattern to determine the number of possible codes at pretty much any step
We can write this out in terms of combinations, too. If N is the number of letters that we're using, then the number of codes can be written as:
N + NC2
So with four letters, we get 4 + 4C2 = 10 different codes.
I hope that helps!
-Carolyn