A common problem in chemistry (and feedlots, and home distilling, etc),
You have two solutions of different concentrations, let's call them H (high concentration) and L (low concentration), and you want to mix them to create a solution of concentration F (final concentration).
Using algebra, it takes a bit of time to get this.
xH + (1-x)L = C, etc, etc, etc, solve for x.
Using the Pearson's Square, it goes like this:
H - F = parts of solution L
F - L = parts of solution H
Gaaahhh.