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?

Alright, so we know that mitosis contains prophase, metaphase, anaphase, and telophase. You can add all the cells in these 4 phases together. 

13 cells in prophase, 12 cells in metaphase, 3 cells in anaphase, and 2 cells in telophase.

13+12+3+2= 30 cells 
So, 30 cells are in mitosis, but don't forget to add the 30 to the already 90 in interphase to find out how many cells there are in all.

30+90=120 cells in all

After this, you just need to divide 30/120 to get simplified 1/4. This means that mitosis takes up 1/4 of the total cell cycle. If you want to find this in hours, just multiply 1/4 by 24 (hours in a day) to get 6.