out of 13 possible books, you plane to take 4 with you on vacation .How many different collection of 4 books can you take?

out of 13 possible books, you plane to take 4 with you on vacation .How many different collection of 4 books can you take?

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This is given by the combinatorial  C(13,4)  .......we're counting the number of different sets we can make by choosing any 4 books of 13.

And C(13,4) = 715

One more note  ...   order within any set doesn't matter...the set of books {A,B,C,D} is the same thing as {A,D,B,C}

another notation is

$$^{13}C_4$$    You can get the answer straight from your calculator.