In an examination of Mathematics paper, there are 20 questions of equal marks and the question paper is divided into three sections : A, B and C. A student is required to attempt total 15 questions taking at least 4 questions from each section. If section A has 8 questions, section B has 6 questions and section C has 6 questions, then the total number of ways a student can select 15 questions is .
Answer
11376
Solution
4 from each section =12; the other 3 questions are shared out (B and C can take at most 2 more each).
(A, B, C) =(7, 4, 4): 8C7 6C4 6C4=8 15 15=1800
(6, 5, 4), (6, 4, 5): 2 28 6 15=5040
(5, 5, 5): 8C5 6C5 6C5=56 6 6=2016
(5, 6, 4), (5, 4, 6): 2 56 1 15=1680
(4, 5, 6), (4, 6, 5): 2 70 6 1=840
total =1800+5040+2016+1680+840
=11376
Answer: 11376