A single cell undergoes mitosis once every 15 minutes. If a single cell is put into a petri dish, how many cells will there be in that dish after an hour?

An hour contains 4 segments of 15 minutes. Therefore the cell will undergo mitosis 4 times. Each time that the cells in the dish undergo mitosis, their number doubles. As such, the single cell in the dish will double in number 4 times.
After 0 minutes: 1 cellAfter 15 minutes: 2 cellsAfter 30 minutes: 4 cellsAfter 45 minutes: 8 cellsAfter 60 minutes: 16 cells
The answer is therefore 16.