Actually - when a cell divides each cell starts out 1/2 as big. So even though the number doubles every minute - the same CAN'T be said about the size. So if the plate was full at 12:00, it was pretty much full at 11:59. Growth and division are not the same thing.
try reading the puzzle first without changing it to fit your own stupidity. This wasn't even a logic problem. They give you the answer right in the question. Anyone who didn't get this one has got some serious IQ issues.