Maths Olympiad Prep

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Combinatorics Difficulty 3.6 AMC 10/12 Find the answer

If 55 girls and 22 boys go to two places to participate in volunteer activities, and both places require both girls and boys, then there are ______ different distribution plans.

A number or a short expression. Spacing and $ signs are ignored.

Solution

To solve this problem, let's break it down into clear, step-by-step calculations:

Step 1: Arrange the 22 boys. Since both places require boys, each boy must go to a different place. There are A22A_2^2 ways to arrange the 22 boys, which is calculated as follows:
A22=2!=2A_2^2 = 2! = 2

Step 2: Arrange the 55 girls, considering that they must be split between two places. There are two scenarios for dividing the girls:

- Scenario 1: Divide the girls into two groups, with one group having 11 girl and the other having 44 girls. The number of ways to select 11 girl from 55 is calculated using combinations:
C51=5!1!(51)!=5C_5^1 = \frac{5!}{1!(5-1)!} = 5

- Scenario 2: Divide the girls into two groups, with one group having 22 girls and the other having 33 girls. The number of ways to select 22 girls from 55 is:
C52=5!2!(52)!=10C_5^2 = \frac{5!}{2!(5-2)!} = 10

Adding the ways from both scenarios for the girls gives us a total of 5+10=155 + 10 = 15 ways to split the girls.

Step 3: Combining the arrangements of boys and girls. Since the boys can be arranged in 22 ways and the girls can be arranged in 1515 ways (the sum from both scenarios), the total number of ways to distribute the 55 girls and 22 boys to two places is:
A22×(5+10)=2×15=30A_2^2 \times (5 + 10) = 2 \times 15 = 30

However, we need to account for the fact that there are 22 places, which has already been implicitly considered in our approach for both boys and girls. Therefore, we don't multiply by 2 again at the end as the initial solution implied; the correct total number of distribution plans is directly the result of our calculation:

3030

Therefore, the correct answer, accounting for the detailed steps and correcting the multiplication error in the final step of the provided solution, is 30\boxed{30}.

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