nishtil wrote:

VeritasPrepKarishma wrote:

nishtil wrote:

Unique letter combinations are 26

two letter combinations are = 26c2 = 26X25/2 = 13X25 = 325

two letter code where two letters are same = 26

total combinations = 26+325-26 = 325

answer should be 325

Would you say AC and CA are different codes or same?

I think you will agree that they are different codes. Hence you do not use the combination formula here (26C2) because it only selects 2 different letters out of the 26 letters.

So how do you solve this question? You can do it in two different ways:

Method 1:

You select 2 different letters and then arrange them.

26C2 * 2! = 650

Method 2:

You use basic counting principle. (Check out:

http://www.veritasprep.com/blog/2011/10 ... inatorics/)

26*25 = 650

The number of one letter codes is 26 so total = 650 + 26 = 676

I agree with you and understood your point. however in one of

kaplan CAT the OA is porvided as 351. Please see the attachment. Please anyone explain why OA is 351

Attachment:

Kaplan CAT.PNG

Yes, because the question in the attachment is different from the main post. It states an additional condition that if a 2 letter code is used, the reverse cannot be used.

So,

AB- Is one valid code

BA- will not be a valid code because AB has already been used.

Whereas according to the main question, both AB and BA will be considered valid codes.

Original post's Answer: \(P^{26}_{1}+P^{26}_{2}\)

Attachment's Answer: \(C^{26}_{1}+C^{26}_{2}\)

_________________

~fluke

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