Enigma 252: Three-point circle
21 January 2015
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From New Scientist #1399, 1st March 1984 [link]
Each of the black dots in the picture is one of the elements — 3 speakers and 22 suppressors — in Uncle Pinkle’s Triphonic Asymmetric Garden Audio-system. The whole square array measures 40 × 40 metres. Uncle Pinkle sits at C, precisely the same distance D from each speaker, but not at distance D from any suppressor; and C must not be directly between any two system-elements, so it isn’t on any of the vertical, horizontal or diagonal lines in the picture. Within these restrictions, Uncle Pinkle has arranged the system-elements and placed his seat so that D is as small as possible.
How far is the distance D? (An answer to the nearest centimetre will do).