In allium, interphase lasts about 15 hours, and mitosis takes up 80 minutes. Assuming that each stage of mitosis takes the same amount of time, how many hours old is a cell that is just
starting anaphase. (Yes, you'll need to do some math here - show your work or explain your
reasoning)

15 hours and 40 minutes

Explanation:

The stages of mitosis are prophase, metaphase, anaphase and telophase. Anaphase is the stage of mitosis after metaphase where replicated chromosomes are segregated to opposite poles of the cell. Anaphase is the third stage after prophase and metaphase, which in this case have the same duration (20 minutes each one). In consequence, a cell that is just starting anaphase is 15 hours and 40 minutes old.

Interphase: 15 hours

Prophase: 20 minutes  

Metaphase: 20 minutes