2007 dat QR question

[myshinyteethandme]

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Hey guys. Number 38 on the QR section of the 07' DAT reads:
Jill has six different books. She will select one book on Monday and a different one to read on Wednesday. In how many ways can Jill select two different books?

The answer is 30 and bootcamp gives the solution of multiplying 6x5. But why wouldnt this be a combination problem where the answer is 15?

THANKS!

 

[NLP26]

It's because on Monday, she will have a pool of 6 books to choose from. On Wednesday, she'll choose one from a pool of 5. So, 6 x 5

 

[KyoPhan]

You are confusing combination with permutation.

Combination - Order doesn't matter
I can select BookA on Mon and BookB on Wed. I can also select BookB on Mon and BookA on Wed. These count as 1 possibility because the end result of what she has is the same.
A B equals B A

Permutation - Order does matter
I can select BookA on Mon and BookB on Wed. I can also select BookB on Mon and BookA on Wed; however the order is different, so they count as 2 possibility. The end result of books she has is the same, but the way she selected it was different.
A B does not equal B A

In this problem, they are asking how many ways the book can be selected. This is a permutation problem where order does matter. The way she selected them does matter. It's not asking about the end result. So n!/(n-r)!

If they reworded it to be something like:
"How many different combination of books can she end up with?" It would be combination.

 

[myshinyteethandme]

You are confusing combination with permutation.

Combination - Order doesn't matter
I can select BookA on Mon and BookB on Wed. I can also select BookB on Mon and BookA on Wed. These count as 1 possibility because the end result of what she has is the same.
A B equals B A

Permutation - Order does matter
I can select BookA on Mon and BookB on Wed. I can also select BookB on Mon and BookA on Wed; however the order is different, so they count as 2 possibility. The end result of books she has is the same, but the way she selected it was different.
A B does not equal B A

In this problem, they are asking how many ways the book can be selected. This is a permutation problem where order does matter. The way she selected them does matter. It's not asking about the end result. So n!/(n-r)!

If they reworded it to be something like:
"How many different combination of books can she end up with?" It would be combination.

Thank you so much!