Worked solution
Separate disjoint cases from inclusion–exclusion
Find the number of arrangements of the 6 letters in the word ELEVEN in which
(i) there are no restrictions,
(ii) all the 3 letters E are consecutive,
(iii) the first letter is E and the last letter is N,
(iv) the first letter is E or the last letter is N or both.
[J1986/I/10]
CJC · Discussion · Question 5
Answer
(i) 120; (ii) 24; (iii) 12; (iv) 68.
(i) Repeated letters
There are three identical E’s and one each of L, V and N. Thus .
(ii) One EEE block
Treat EEE, L, V and N as four objects. The E’s have only one internal order, giving .
(iii) Fix both endpoints
Fix E first and N last. The four middle letters are E, E, L and V, so their arrangements number .
(iv) Method 1: three disjoint cases
First E and last not N: if the last letter is L or V, there are arrangements of the remaining middle letters; if it is E, there are . This case totals 48. First not E and last N: the first letter is L or V, giving . First E and last N gives 12 from (iii). The disjoint total is .
(iv) Method 2: inclusion–exclusion
First E gives arrangements of E, E, L, V, N in the remaining positions. Last N gives arrangements of E, E, E, L, V. Their intersection, first E and last N, was counted twice and has size 12. Hence .
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Common mistakes
The two endpoint conditions overlap. Adding their individual counts without correction double-counts part (iii).
Solution notes
All printed asks, working, counting factors and source annotations visually compared and retained. No marks are printed for this question.