Example No. 7: Permutations and Combinations
Problem:
In how many ways can 5 boys and 3 girls be seated in a row such that no two girls are together?
Solution:
Step 1: Find the total number of ways
We have a total of 8 people who need to be seated in a row. The number of ways to arrange them is:
8! = 40,320
Step 2: Find the number of ways with two girls together
If two girls sit together, we can consider them as one entity. Then we have 7 entities to arrange in a row, which can be done in:
7! × 2! = 10,080
The factor of 2! accounts for the number of ways to arrange the two girls within their entity.
Step 3: Find the number of ways with three girls together
If three girls sit together, we can also consider them as one entity. Then we have 6 entities to arrange in a row, which can be done in:
6! × 3! = 2,160
The factor of 3! accounts for the number of ways to arrange the three girls within their entity.
Step 4: Subtract the number of ways with two or three girls together from the total number of ways
The number of ways with two or three girls together is:
10,080 + 2,160 = 12,240
So the number of ways with no two girls together is:
40,320 - 12,240 = 28,080
Therefore, there are 28,080 ways to seat 5 boys and 3 girls in a row such that no two girls are together.