Permutations and Combinations Question from PrepSwift

GRE Quant Problem Solving
1

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Hi!
I am going through the Prepswift Data Analysis Section, specifically the parts on Permutations and Combinations. I get confused when trying to figure out which formula to use in the example above. On one level, I understand we can follow the thought process:
10 books, 4 choices => order doesn’t matter => use the combination formula

But on the other hand, if I was solving this on my own I would have went:
“Okay, 10 options for the first book, 9 remaining options for the second book, 8 for the 3rd book, and 7 for the last book => 1098*7 => 5040”

Although logically I understand how I can plug the numbers in the correct way like in the attached image, I don’t really understand why the process I wrote in the second paragraph doesn’t work. Would anyone be able to explain how this is wrong? Thanks in advance :slight_smile:

2

Does the order of the four books matter?

3

Ah okay. So it’s wrong because we’re just looking at the selection of 4 books as a whole, and the order we select them in is just not a part of the problem - it’s not like creating a password or anything where the order matters. I think I understand a little better now. Thanks :slight_smile:

4

Correct.