SITUATION:
20 students in class.
There are 10 benches with each bench able to sit two students.
A test were given and student scored 12 A's and 8 B's
8 A's from same bench and 4 A's from students sitting at different bench.
4 B's from the same bench and 4 B's from students sitting at different bench.
What is the probability of this happening (8 A's and 4 B's from the same bench given 12 A's and 8 B's)?