Bunuel
In how many ways can letters the word ATTITUDE be rearranged such that no two Ts are adjacent to each other?
A. 1800
B. 2400
C. 3600
D. 4320
E. 6720
Take the task of seating the letters and break it into
stages.
Stage 1: Arrange the non-T letters (A, I, U, D and E)
Since n unique objects can be arranged in n! ways, we can arrange these five letters in 5! ways. In other words, we can complete stage 1 in
120 ways.
Key step: For each arrangement of the 5 non-T letters, add a space on each side of each letter. So, for example, if we add spaces to the arrangement ADIUE, we get: _A_D_I_U_E_ Each of these six spaces represents a possible location for each of the 3 T's. Notice that this configuration ensures that no T's can be adjacent.
Stage 2: Place T's in 3 of the 6 available spaces .
IMPORTANT: Since we're placing 3
identical T's, the order in which select the 3 spaces doesn't matter.
So, we can use combinations.
We can choose 3 of the 6 spaces in 6C3 ways (=
20 ways)
So, we can complete stage 2 in
20 ways.
At this point, we'll throw away the remaining spaces, leaving an arrangement with all 8 letters.
By the Fundamental Counting Principle (FCP), the number of ways to complete the 2 stages (and thus place all 7 letters) = (
120)(
20) = 2,400 ways.
Answer: B
Cheers,
Brent
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