Assuming hexagonal packing, a 0.750 inch in diameter penny [1] is going to "cover" the area of a ...

Assuming hexagonal packing, a 0.750 inch in diameter penny [1] is going to "cover" the area of a hexagon in which the penny is inscribed. That area is sqrt(3)/2 * (0.750 in)^2 [2]. So the cost per area is 1 cent / (sqrt(3)/2 * (0.750 in)^2) = $2.956 / sqft [3].
I'm guessing the author of the caption was using rectilinear packing with a gap of 0.25 inches between the pennies.

[1] http://en.wikipedia.org/wiki/Penny_(United_States_coin)
[2] http://en.wikipedia.org/wiki/Hexagon
[3] http://www.wolframalpha.com/input/?i=1+cent+%2F+%28sqrt%283%29%2F2+*+%280.750+in%29%5E2%29

Syndicated 2012-03-12 14:53:02 from Plus Public Activity Feed for David Barksdale