Maths Olympiad Prep

Track / Stage 3 / 165 of 260 #165 of 1964

Problem 165

AMC 10/12, early questions
Combinatorics Difficulty 3.4 Find the answer

There are 55 different books to be distributed among three students, with each student receiving at least 11 book and at most 22 books. The number of different distribution methods is ________\_\_\_\_\_\_\_\_.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Official solution

Analysis

This problem involves the combined application of combinations and permutations. We can first determine that the distribution can be divided into 22, 22, and 11 books.

Step-by-step solution

1. First, we will find the number of ways to choose 22 books out of 55 for the first two students who will receive 22 books each. This can be calculated using the combination formula: C52=5!2!(52)!=10C_5^2=\dfrac{5!}{2!(5-2)!}=10.

2. Next, we have 33 books remaining (as the first two students have already received their books), and we will choose 22 books for the second student who will receive 22 books. Again, using the combination formula: C32=3!2!(32)!=3C_3^2=\dfrac{3!}{2!(3-2)!}=3.

3. Now, we have considered the distribution of books to the first two students receiving 22 books each. However, since there are two students receiving the same number of books, we need to divide the result by the number of arrangements of these students (A22=2A_2^2 = 2).

4. Finally, there is only 11 book left, and it will be given to the last student. There are A33=3A_3^3 = 3 ways to choose this last student.

5. Combining these results, we find the total number of distribution methods: C52C32A22A33=10323=90\frac{C_5^2 C_3^2}{A_2^2} \cdot A_3^3 = \frac{10 \cdot 3}{2} \cdot 3 = 90

Therefore, there are 90\boxed{90} different methods to distribute the books.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.