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June 2015 p62 q6
2714
Find the number of different ways the 7 letters of the word BANANAS can be arranged
if the first letter is N and the last letter is B,
if all the letters A are next to each other.
Solution
(i) With the first letter as N and the last letter as B, the arrangement is N****B. We have 5 letters left: A, A, A, N, S. The number of ways to arrange these 5 letters is given by:
\(\frac{5!}{3!} = 20\)
since there are 3 A's which are identical.
(ii) If all the letters A are next to each other, treat them as a single unit (AAA). The letters to arrange are B, (AAA), N, N, S. The number of ways to arrange these 5 units is: