Serial Dilution
What about the cell culture? How much will you have to dilute the stock solution to get it down to 103 cells/mL?
Try it yourself and then put the cursor in the black box to see the answer.
x = 1 mL (2 x 108 cells/mL )/ (103 cells/mL) = 2 x 105 mL
dilute the stock by 1: 2 x 105