I don't know if I'm doing this right.
Suppose you have 12 marbles. 5 are red and 7 are blue. If you randomly choose 4 marbles at once, what is the probability you will get exactly 3 red marbles?
I did the equation: $(5/12)(4/11)(3/10)(7/9)$
Then I tried to do it in combination form: ${5 \choose 3}$${7 \choose 1}$ divided by ${12 \choose 4}$. But it doesn't yield the same result.
Did I do something wrong in the first equation? Or set up my combinations wrong?