We have a solution already made: it is called the stock solution.

We want to use it to make another solution of the same type but more diluted.

The question is how much should I dilute the stock solution to make it?

The magic formula to figure it out is the following:

vol _new x conc _new = vol _st x conc _st

Vol _new : volume of the new solution
conc _new : concentration of the new solution
vol _st : volume of the stock solution that will be part of the new solution and that I have to pipet in a new container
conc _st : concentration of the stock solution