How many ounces of a 40% silver nitrate solution must be mixed with 8 ounces of a 15% silver nitrate solution to make a 30% solution?

Let's say we have to mix "x" ounces of 40% silver nitrate, now the solution is 40% of silver nitrate, so 40% in those "x" oz is silver nitrate, the rest is hmmm water or so.  So how much silver nitrate is there in "x" anyway?  (40/100) * x, or 0.4x.

we know in 8 oz of 15% silver nitrate, there's only 15% of silver nitrate, the rest is other substances, how much is that?  (15/100) * 8, or 1.2.

our mixture will be 30% of silver nitrate, and hmmm say we end up with "y" ounces, how much silver nitrate is in the "y" oz?  (30/100) * y, or 0.3y.

we also know that, whatever "x" and "y" are, we know that x + 8 = y  and that 0.4x + 1.2 = 0.3y

 

The volume is 56.8 triangular the