What are the two equations you can set up in any solution problem? For example, mixing some percent of acid with water.
The two equations are (1) the volume equation, which concerns total amount of liquid, and (2) the solute equation, which concerns the amount of the stuff dissolved.
For example, liters of water plus liters of original acid would equal liters of the resulting solution. That's the volume equation.
For the solute equation, we know that amount of solute = (concentration)(V of solution)
Here, we have to express the concentration, often given as a percent, in decimal form.
Suppose the original acid is a 80% acid and the final acid is a 30% acid. Then, solute in the original acid is:
0.8(V of original acid)
and solute in the resulting solution is
0.3(V of resulting solution)
and the solute equation sets those two quantities equal. With two simultaneous equations, you can solve.