A Malaria Puzzler:
Pretend you are a scientist researching malaria. You have a 10 milliliter (ml) solution of red blood cells at a concentration of 10 cells/ ml. (This means you have 10 red blood cells in every ml of solution.) However, you want to do an experiment with your cells that requires 8 ml of solution at a concentration of 5 cells/ ml. How will you dilute the concentration of your original solution to get 8 ml of solution with a concentration of 5 cells/ ml?
---> 
= ?
(Hint: How many ml of the 10 cells/ ml solution should you use?)
Also, check out the answers to the last puzzler from Eddie Moler!