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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 6!/3!=1206!/3!=120.

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(ii) One EEE block

Treat EEE, L, V and N as four objects. The E’s have only one internal order, giving 4!=244!=24.

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(iii) Fix both endpoints

Fix E first and N last. The four middle letters are E, E, L and V, so their arrangements number 4!/2!=124!/2!=12.

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(iv) Method 1: three disjoint cases

First E and last not N: if the last letter is L or V, there are (21)(4!/2!)=24\binom21(4!/2!)=24 arrangements of the remaining middle letters; if it is E, there are 4!=244!=24. This case totals 48. First not E and last N: the first letter is L or V, giving (21)(4!/3!)=8\binom21(4!/3!)=8. First E and last N gives 12 from (iii). The disjoint total is 48+8+12=6848+8+12=68.

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(iv) Method 2: inclusion–exclusion

First E gives 5!/2!=605!/2!=60 arrangements of E, E, L, V, N in the remaining positions. Last N gives 5!/3!=205!/3!=20 arrangements of E, E, E, L, V. Their intersection, first E and last N, was counted twice and has size 12. Hence 60+20−12=6860+20-12=68.

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Six positions for ELEVEN with E fixed first and N fixed last. The middle four positions contain E,E,L,V and have 4!/2!=12 arrangements.
Six positions for ELEVEN with E fixed first and N fixed last. The middle four positions contain E,E,L,V and have 4!/2!=12 arrangements.
Six positions for ELEVEN with E fixed first and N fixed last. The middle four positions contain E,E,L,V and have 4!/2!=12 arrangements.

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.

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