Permutations or combinations? Combinations Answer: 220
Practice makes perfect
We want to find how many ways there are to select 9 out of 12 books from the list. Note that, in this case, the order is not important. Therefore, the situation involves combinations. To calculate the answer, we will use the formula for combinations of n objects taken r at a time.
_n C_r = _n P_r/r! ⇔ _n C_r = n!/(n-r)!r!
In our exercise, n= 12 and r= 9. Let's substitute these values into the above formula.
