permutations

If a committee of 6 members is chosen from 4 teachers and 6 students, what is the probability that it contains 2 teachers and 4 students. Can someone please explain to me why we have to divide the answer by 10C6??

I just started learning this topic so please don't laugh at me if you find this question easy but I can't do any of these ..... Can you guys please help me out with this entire question??