The circumference of a circle of diameter d is given by
... C = π·d
Chris will need π·(18 ft) ≈ 56.55 ft of fence. S/he will need to buy 57 ft of fence.
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Whether you use 57 because 56.55 rounds up to that, or because 56.55 is needed and 56 would be too short, doesn't really matter in this case. I tend to choose the latter reason.
