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Difficulty: 3/102024 NMTC 2024 (QII-61)

In group of 8 people, how many ways can you choose committee of 3 people if two specific individuals refuse to work together?

Guide / Hint

Hint 1: Calculate the total number of ways to choose 3 people from 8 without any restrictions.

Hint 2: Count how many of these committees are invalid because they contain both of the conflicting individuals.

Hint 3: Subtract the invalid combinations from the total combinations to find the answer.

Solution

Step 1 (Total Unrestricted Combinations): We want to choose a committee of 3 people from a group of 8. Without any restrictions, the number of ways is:

Step 2 (Identify Invalid Combinations): Let the two individuals who refuse to work together be and . A committee is invalid if it contains both and .

  • Since and occupy 2 seats in the 3-person committee, we must choose exactly 1 more person from the remaining people to fill the last seat.

  • The number of invalid committees is:

Step 3 (Calculate Valid Combinations): We subtract the invalid committees from the total unrestricted committees:

Step 4 (Conclusion): The committee can be chosen in exactly 50 different ways.

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    2024 NMTC 2024 QII-61 - Olympiad Math Olympiad Question | Leminno