Carcass wrote:
In how many ways can the letters of the word SPENCER be arranged if the S and P must always be together and the N and C must always be together?
A. 12
B. 24
C. 60
D. 120
E. 240
Take the task of arranging the letters and break it into
stages.
Stage 1: Glue S and P together.
Note, this will ensure that S and P are together.
There are 2 ways to glue S and P together: SP and PS
So we can complete stage 1 in
2 ways
Stage 2: Glue N and C together.
There are 2 ways to glue N and C together: NC and CN
So we can complete stage 2 in
2 ways
IMPORTANT: We now have 5 "objects" to arrange. They are: E, E, R, S/P combo, N/C combo
Stage 3: arrange the 5 "objects" in a row
--------ASIDE-------------------------------
When we want to arrange a group of items in which some of the items are identical, we can use something called the MISSISSIPPI rule. It goes like this:
If there are n objects where A of them are alike, another B of them are alike, another C of them are alike, and so on, then the total number of possible arrangements = n!/[(A!)(B!)(C!)....] So, for example, we can calculate the number of arrangements of the letters in MISSISSIPPI as follows:
There are
11 letters in total
There are
4 identical I's
There are
4 identical S's
There are
2 identical P's
So, the total number of possible arrangements =
11!/[(
4!)(
4!)(
2!)]
---------BACK TO THE QUESTION--------------------
In stage 3, we must arrange 5 objects: E, E, R, S/P combo, N/C combo
There are
5 objects in total
There are
2 identical E's
So, the total number of possible arrangements =
5!/[(
2!)] =
60By the Fundamental Counting Principle (FCP), we can complete all 3 stages (and thus arrange all of the letters) in
(2)(2)(60) ways (= 240 ways)
Answer: E