Answer:
C = 0.75(n/2)
Step-by-step explanation:
For each pair of signs, $0.75 is needed for the posterboard.
C = 0.75(n/2)
This can be simplified to ...
C = 0.375n . . . . for n an even number
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If posterboard only comes in the size given, then the more technically correct answer is ...
C = 0.75·ceiling(n/2)