Could someone please help me with this formula, I'm sure it's pretty simple but I don't know how to do it.

What I need is a formula for dilution.

I have 10 ml of liquid that is at a strength of 500 ppm.

How would I make a formula to figure out how much liquid to add that has a strength of 0 ppm to the 10 ml of liquid that has 500 ppm, to come out with a liquid of 4 ppm?

For example if I add 90 ml of 0 ppm liquid and to this liquid I add 10 ml of 500 ppm water - I would have a final liquid that was 100 ml in size with a strength of 50 ppm.

Thanks,

Bill