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?
Problem 137
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:
Second case (C for chess, B for Go):
The number of ways to select 1 person from C and 1 from B is:
Third case (A for chess, C for Go):
The number of ways to select 1 person from A and 1 from C is:
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:
Adding all these up, the total number of distinct ways to choose the 2 students is:
Therefore, there are different ways to choose the students for the chess and Go competitions.