From a group of 3 freshmen, 4 sophomores, 4 juniors and 3 seniors a committee of size 4 is randomly selected. Find the probability that the committee will consist of

a)      1 from each class

b)      2 sophomores and 2 juniors

c)      Only sophomores and juniors

http://www.bilkent.edu.tr/~math250/Solution%20set1.pdf