For any chemists out here, a question. It's been a long while since I did any dilution calculations and want to make sure I haven't lost my marbles.

So I have 1 liter containing 40,000 ppm of some substance X.

I combine this with 3 liters containing 1,000 ppm of substance X.

This results in what?

4 liters containing: (1 x 40,000 + 3 x 1,000) / (1 + 3) =

4 liters containing 10,750 ppm of substance X. Right?

Or have I lost the plot here?