For this you will need four pennies. Arrange the pennies so that the centers of all four are equidistant from each other.
Example:
(1) (2) (3) (4)
This will not work because penny 1 and penny 4 are farther apart than penny 1 and penny 2.
If we take a 3D pyramid, the centres of the pennies at the base will be equidistant (equilateral triangle).
Also, the penny at the top will also be equidistant from each of the 3 other pennies(each of the sides will form an equilateral triangle).

Posted by Sparsh
on 20060503 09:26:27 