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June 2012 p63 q3
2724
In how many ways can all 9 letters of the word TELEPHONE be arranged in a line if the letters P and L must be at the ends?
Solution
The word TELEPHONE consists of 9 letters, with the letters P and L fixed at the ends. This leaves us with 7 letters: T, E, E, E, H, O, N to arrange in the middle.
First, calculate the number of ways to arrange these 7 letters. Since there are 3 E's, the number of distinct arrangements is given by:
\(\frac{7!}{3!}\)
Calculating this gives:
\(\frac{7!}{3!} = \frac{5040}{6} = 840\)
Since P and L can be at either end, we have 2 possible arrangements for the ends (P at the start and L at the end, or L at the start and P at the end). Therefore, multiply the number of arrangements by 2:
\(840 \times 2 = 1680\)
Thus, the total number of ways to arrange the letters is 1680.