Olympiad Maths Prep

Track / Stage 3 / 137 of 260 #137 of 2000

Problem 137

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

There are 9 students, among which 2 can play chess but not Go, 3 can play Go but not chess, and 4 can play both. If we want to select 2 students from these 9, one for a chess competition and another for a Go competition, how many different ways are there to choose them?

Official solution

Let set A represent the 2 students who play chess but not Go, set B represent the 3 students who play Go but not chess, and set C represent the 4 students who can play both. The method to choose 2 students for the competitions can be broken down into the following 4 cases:
- Select 1 person from set A for the chess competition and 1 person from set B for the Go competition.
- Select 1 person from set C for the chess competition and 1 person from set B for the Go competition.
- Select 1 person from set C for the Go competition and 1 person from set A for the chess competition.
- Select 2 people from set C, one for each competition.

Now, let's calculate each scenario:

First case (A for chess, B for Go):
The number of ways to select 1 person from A and 1 from B is:
A×B=2×3=6 |A| \times |B| = 2 \times 3 = 6

Second case (C for chess, B for Go):
The number of ways to select 1 person from C and 1 from B is:
C×B=4×3=12 |C| \times |B| = 4 \times 3 = 12

Third case (A for chess, C for Go):
The number of ways to select 1 person from A and 1 from C is:
A×C=2×4=8 |A| \times |C| = 2 \times 4 = 8

Fourth case (2 people from C):
The number of ways to select 2 people from C for the two competitions is equivalent to the number of ways to choose 2 different people from a group of 4, which can be calculated as a combination:
(C2)=(42)=4×32×1=6 \binom{|C|}{2} = \binom{4}{2} = \frac{4 \times 3}{2 \times 1} = 6

Adding all these up, the total number of distinct ways to choose the 2 students is:
6+12+8+6=32 6 + 12 + 8 + 6 = 32

Therefore, there are 32\boxed{32} different ways to choose the students for the chess and Go competitions.

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