Abiologist examines a series of cells and counts 90 cells in interphase, 13 cells in prophase, 12 cells in metaphase, 3 cells in anaphase, and 2 cells in telophase. if a complete cycle for this type of cell requires 24 hours, what is the average duration of mitosis?

unfortunately not a multiple choice

 Mitosis involves Prophase, Metaphase, Anaphase, and Telophase. 

So you have 
13 + 12 + 3 + 2 = 30 cells in mitosis 
and 
90 + 30 = 120 cells in total 

Therefore your ratio of cells in mitosis to total cells is 30 / 120 = 1 / 4. That means mitosis should take up roughly 1/4 of the total cell cycle length. Since the completely cycle takes 24 hours, mitosis would then take (1/4)*24 = 6 hours

Because the carnivores keep the herbivore population in check