You start out with a cell suspension of 5e6 cells in a 10ml. you want to seed a new flask with half a million cells. how many milliliters of the cell suspension do you transfer to the new flask?

5e6 cells in 10mL means we have 5 million cells in this volume. 

We can figure out this by cross-multiplication:
5.000.000 cells ==> 10 mL
500.000 cells ==> x mL
so 500.000 x  = 1 mL.

So you take 1 milliliter of your initial flask and transfer it to the new one.

They both determine the exact age of rocks.