Porisma

Worked solution

Rearrange the ACE icon with repeated symbols

A student wishes to re-arrange the letters and asterisks on the icon shown. Given that the letters of the word ACE must be together in the clockwise manner, find the number of possible arrangements if (i) the asterisks must be separated, (ii) the positions of the letters and asterisks are labelled.

Expand diagram

Use the scrollbars or arrow keys to inspect details.

Eleven symbols around a ring, clockwise from S at the top: S,D,F,F,G,G, asterisk,A,C,E, asterisk. Exactly one each of A,C,E,S,D; two F, two G, two asterisks. The question requires A,C,E together in that clockwise order.
Eleven symbols around a ring, clockwise from S at the top: S,D,F,F,G,G, asterisk,A,C,E, asterisk. Exactly one each of A,C,E,S,D; two F, two G, two asterisks. The question requires A,C,E together in that clockwise order.
Eleven symbols around a ring, clockwise from S at the top: S,D,F,F,G,G, asterisk,A,C,E, asterisk. Exactly one each of A,C,E,S,D; two F, two G, two asterisks. The question requires A,C,E together in that clockwise order.

CJC · Lecture · Example 19

Inventory the symbols and fix the block order

There is one each of A,C,E,S,D, two F copies, two G copies and two asterisks: 11 symbols. Treat consecutive clockwise A,C,E as one block. This block has only one allowed internal order; do not multiply by 3! or 2!.

How did this part go?

(i) Arrange the seven non-asterisk objects

The ACE block,S,D,F,F,G,G are seven objects. Fix the unique ACE block as a circular reference and arrange the other six, correcting for two identical F copies and two identical G copies: (7−1)!/(2!2!)=180(7-1)!/(2!2!)=180.

How did this part go?

(i) Choose two different gaps

Those seven circular objects create seven gaps, including the gaps immediately before and after the ACE block but none inside it. Put the two identical asterisks in two different gaps, giving (72)=21\binom72=21 choices. Total [(7−1)!/(2!2!)](72)=180×21=3780[(7-1)!/(2!2!)]\binom72=180\times21=3780.

How did this part go?

(ii) Include both asterisks among nine outer objects

Part (ii) is a separate condition; it does not retain the separation requirement of part (i). The ACE block and eight remaining symbols give nine outer objects. Fix ACE as a reference and correct for pairs of F,G and asterisks: (9−1)!/(2!2!2!)=5040(9-1)!/(2!2!2!)=5040 circular arrangements.

How did this part go?

(ii) Restore the eleven labeled symbol positions

There are 11 choices for the labeled position occupied by A, which fixes the clockwise ACE block and anchors the circular order. Hence [(9−1)!/(2!2!2!)]×11=55,440[(9-1)!/(2!2!2!)]\times11=55{,}440. Use 11 original symbol positions, not nine compressed objects; ACE occupies three consecutive positions and may wrap across the label boundary.

How did this part go?

Compare your sketch

Expand diagram

Use the scrollbars or arrow keys to inspect details.

Part (i): ACE block,S,D,F,F,G,G make seven circular objects with seven gaps; insert the two identical asterisks into two distinct gaps. Part (ii): ACE block,S,D,F,F,G,G, asterisk, asterisk make nine circular objects; the eleven original symbol positions are labeled. The block spans three symbol positions. Repeated Fs, Gs, and in part (ii)asterisks give factorial divisors. ACE internal order is fixed.
Part (i): ACE block,S,D,F,F,G,G make seven circular objects with seven gaps; insert the two identical asterisks into two distinct gaps. Part (ii): ACE block,S,D,F,F,G,G, asterisk, asterisk make nine circular objects; the eleven original symbol positions are labeled. The block spans three symbol positions. Repeated Fs, Gs, and in part (ii)asterisks give factorial divisors. ACE internal order is fixed.
Part (i): ACE block,S,D,F,F,G,G make seven circular objects with seven gaps; insert the two identical asterisks into two distinct gaps. Part (ii): ACE block,S,D,F,F,G,G, asterisk, asterisk make nine circular objects; the eleven original symbol positions are labeled. The block spans three symbol positions. Repeated Fs, Gs, and in part (ii)asterisks give factorial divisors. ACE internal order is fixed.

Common mistakes

Do not permute the letters inside ACE. Part (i) uses unlabelled positions with separated asterisks; part (ii) uses labelled positions without that extra separation restriction. Repeated copies require the stated divisors. The unique ACE block anchors the rotational count, avoiding an unjustified general division rule for arbitrary repeated circular objects.

Read the hint