1 boy and 6 girls are arranged in a row with 7 chairs marked A-G. How ...
Since order matters in this question (Seats B, C and D occupied with Jill, Greta and Pauline is a different seating arrangement from Seats B, C and D occupied with Greta, Pauline and Jill), we can solve it using either the Filling Spaces method or the Permutation formula.
Step 1: Understand the objective
There are 7 people in this question – 1 boy and 6 girls.
And, there are 7 chairs.
Each chair is unique, because it is marked with a different number.
In this question, two cases are possible:
Case 1: The boy sits on Chair A and the 6 girls are arranged on chairs B-G
Case 2: The boy sits on Chair G and the 6 girls are arranged on chairs A-F
The question here wants us to find the total number of seating arrangements in which the boy sits either on the chair A or on chair G. That is, the question wants us to find the total number of ways in which the 6 girls can be arranged either on chairs B-G or on chairs A-F. This is the objective of the question.
Step 2: Write the objective equation enlisting all tasks
The objective here consists of two tasks:
- Task 1 – Arrange the 6 girls on the 6 seats marked B-G
- Task 2 – Arrange the 6 girls on the 6 seats marked A-F
Next, we need to determine what sign to put in the objective equation – multiplication or addition.
Let’s look at the objective statement again:
“. . . to find the total number of ways in which the 6 girls can be arranged either on chairs B-G or on chairs A-F”
The objective statement contains the words ‘Either. . . Or’
This means that we will put an addition sign between the number of ways of doing the two tasks.
Thus, the objective equation will be:
Now, we know that
The number of ways in which 6 girls can be arranged in 6 seats = 6P6
So, the objective equation becomes:
(Number of arrangements in which the boy sits on chirs A or G) = 6P6 + 6P6
That is,
(Number of arrangements in which the boy sits on chirs A or G) = 2 (6P6)
Step 3: Determine the number of ways of doing each task
In Step 3, using the Permutation Formula (nPn = n!), we get that
6P6 = 6! = 6*5*4*3*2*1 = 720
Step 4: Calculate the final answer
By putting these values in the objective equation, we get:
(Number of arrangements in which the boy sits on chirs A or G) = 2 x720 = 1440
Looking at the answer choices, we see that Option C is correct.