Here's the question you clicked on:
fatboy12
Find the probability of randomly selecting three science books and four history books from a box containing five science books and six history books.
@hba Why would we be adding in this case? It's not "three science books OR four history books"
No problem, I wasn't even sure that you were wrong, I was just confused.
My guess is \(\binom{5}{3} \times \binom{6}{4} \div \binom{11}{7} \) It's assuming they meant to say "choosing 7 books randomly". Because otherwise it could be 100% or even 0%.
The answer would be 5/11 then ?
If I am correct then wolfram agrees: http://www.wolframalpha.com/input/?i=%285+choose+3%29+*+%286+choose+4%29++%2F+%2811+choose+7%29
hmm,I am really sorry about the mistake i made :/