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How many ways can we arrange the word PARALLEL that the vowels are kept together?
A) 120
B) 144
C) 288
D) 360
E) 600
Take the task of arranging the letters and break it into
stages.
Stage 1: "glue" the three vowels (A, A and E) together.
There are three possible outcomes: AAE, AEA and EAA
So, we can complete stage 1 in
3 ways
This ensures that the three vowels are kept together.
NOTE: We now have the following 6 objects to arrange: P, R, L, L, L and the glued-together-vowelsStage 2: Arrange the following 6 objects: P, R, L, L, L and the glued-together-vowels
Notice that 3 of our objects are
identical L's
----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!)]
-----------------------
In P, R, L, L, L and the glued-together-vowels:
There are
6 objects in total
There are
3 identical L's
So the total number of possible arrangements =
6!/(
3!) = 120
So, we can complete stage 2 in
120 ways
By the Fundamental Counting Principle (FCP), we can complete the 2 stages (and thus arrange the letters) in
(3)(120) ways (= 360 ways)
Answer: D
Note: the FCP can be used to solve the MAJORITY of counting questions on the GMAT. So, be sure to learn it.
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