A penny lies at the bottom of a water fountain at a depth of 30 cm. Determine

Question:

A penny lies at the bottom of a water fountain at a depth of 30 cm. Determine the diameter of a piece of paper which, if placed to float on the surface of the water directly above the penny, would totally obscure the penny from view. Treat the penny as a point and assume that n = 1:33 for water.