katzzzz wrote:
Hi, do we need to account for the restriction that impose by the structure as C-V-C-V-C? as one V need to follow a C and we cant do C-C-C-V-V?
I am confused here..
And on top of it, when is it good to use the formula of combination and when we just use the method applied in this question ( like a number lock), thought believe that its the same concept?
Thank you..
We are accounting for it by calculating only the number of ways of writing CVCVC. So the other arrangements of 3Cs and 2Vs are ignored.
You can write the first C in 3 ways.
You can write the next letter V in 2 ways.
The next letter is again C for which we again have 3 options (note that repetition of letters is not a problem)
The next letter V can be chosen in 2 ways.
The last letter C can be chosen in 3 ways again.
This gives us 3*2*3*2*3 = 108 ways.
You use the combination formula only when you have to select a few things out of many things. Here, no selection is required. Say, if there were 10 consonants and we had to make the nouns using 3 DISTINCT consonants, then we would have SELECTED 3 of the 10 (in 10C3 ways) and then arranged them in 3 places in 3! ways.
The method used in this question is the basic counting principle. It is used when you have distinct places for things. I suggest you to check out these posts:
http://www.veritasprep.com/blog/2011/10 ... inatorics/http://www.veritasprep.com/blog/2011/11 ... binations/ _________________
Karishma
Veritas Prep GMAT Instructor
Learn more about how Veritas Prep can help you achieve a great GMAT score by checking out their GMAT Prep Options >