danlew
Sam and Jane meet each other at a party. They have an argument in the party and do not talk to each other for the rest of the party. A photographer wants to take a picture of all the guests at the party. How many options does the photographer have to arrange the 10 guests at the party, if Jane does not want to stand next to Sam when the picture is taken?
A. 9!
B. 8!x9
C. 8x9!
D. \(\frac{10!}{2!}\)
E. 10!
We can use the following equation:
(Total number of ways to arrange the guests) = (number of ways with Jane next to Sam) + (number of ways with Jane not next to Sam)
Thus:
(Number of ways with Jane not next to Sam) = (total number of ways to arrange the guests) - (number of ways with Jane next to Sam)
The total number of ways to arrange the 10 guests in a row is 10P10 = 10!.
To determine the number of ways to arrange the guests when Sam and Jane are next to each other, consider Sam and Jane as a single ‘person.’ Thus, it is like placing only 9 individuals in line, and this is done in 9! ways. However, we also have to consider that Sam and Jane, when next to each other, can be ordered either as [S-J] or as [J-S], and so we must multiply the previous value by 2. Thus, the total number of ways to arrange the 10 guests when Sam is next to Jane is 9! x 2.
Thus, the number of ways with Jane not next to Sam = 10! - 9! x 2 = 10 x 9! - 9! x 2 = (10 - 2) x 9! = 8 x 9!.
If the arithmetic in the previous paragraph is confusing, let’s begin with 10! - 9! x 2.
Note that 10! can be re-expressed as 10 x 9!.
10 x 9! - 9! x 2
Rearrange to get 9! x 10 - 9! x 2.
Now pull out the common factor 9! to get 9!(10 - 2) = 9! x 8, or 8 x 9!
Answer: C