At 12:01, a virus is placed in a cell of a grid of unit equilateral triangles. Every minute, each cell with exactly one adjacent infected cell simultaneously becomes infected with a copy of the virus. (Two cells are adjacent if they share an edge.) The infected cells at 12:01, 12:02, and 12:03 are shown below. Let T be the number of infected cells at 12:05 and P be the perimeter of the infected region at 12:05. Find P−T.

Guest Nov 7, 2021