#### davidtrinh

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my attempt

I do seven choose four to get 7C4 = 35. But, the answer key says 210.

How do you get the answer? Thanks a lot.

- Thread starter davidtrinh
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my attempt

I do seven choose four to get 7C4 = 35. But, the answer key says 210.

How do you get the answer? Thanks a lot.

- Joined
- Nov 24, 2012

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\(\displaystyle \displaystyle {7 \choose 4}=35\)

Now, you need to observe that there are 4! ways to order the 4 books chosen...so the final answer is?

Another way to approach this is to think of the fact that there are 7 choices for the first position, 6 for the second, 5 for the third and 4 for the fourth:

\(\displaystyle N=7\cdot6\cdot5\cdot4\)

But...this returns a number 4 times as large as that you've cited as being the answer.

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Since you are arranging, not just selecting, the correct answer is not

my attempt

I do seven choose four to get 7C4 = 35. But, the answer key says 210.

How do you get the answer? Thanks a lot.

It looks like they may have accidentally done 7!/4!, which is easy to do if you aren't thinking.

- Joined
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