Bunuel
A team of 8 students goes on an excursion, in two cars, of which one can seat 5 and the other only 4. If internal arrangement inside the car does not matter then the number of ways in which they can travel, is
(A) 91
(B) 122
(C) 126
(D) 3920
(E) 4120
Solution:Let’s first assume that the students are indistinguishable. Then the problem becomes: How many ways can 2 positive integers (where the first addend can’t be more than 4 and the second can’t be more than 5) add up to 8? We can see that we can have:
3 + 5 = 8 and 4 + 4 = 8
Of course, the students (and in general, people) are distinguishable, so we have to consider each of the two cases above:
Case 1: 3 + 5 = 8
That is, the first car gets 3 students and the second 5 students. Since the students are distinguishable, the number of choices the first car has is 8C3 and the number of choices the second car has is 5C5 (after 3 students have been chosen to sit in the first car). Therefore, the number of ways the 8 students can be distributed into 2 cars in this case is 8C3 x 5C5 = (8 x 7 x 6)/(3 x 2) x 1 = 8 x 7 = 56.
Case 2: 4 + 4 = 8
That is, the first car gets 4 students and the second 4 students. Since the students are distinguishable, the number of choices the first car has is 8C4 and the number of choices the second car has is 4C4 (after 4 students have been chosen to sit in the first car). Therefore, the number of ways the 8 students can be distributed into 2 cars in this case is 8C4 x 4C4 = (8 x 7 x 6 x 5)/(4 x 3 x 2) x 1 = 2 x 7 x 5 = 70.
Thus, the total number of ways in both cases is 56 + 70 = 126.
Answer: C