Universal dilution and mixing two solutions calculator:

applicable for mixing two solutions with various concentrations  (molar or %, or other), as well as for various pH or mixing with solvent like water

What is molar concentration of solution prepared by mixing 100 mL 10% and 130 mL of 15% sodium hydroxide?
There is nothing difficult in the question, let's try solution mixer:

"Stock concentration:" - 15;
"Solvent concentration:" - 10,
"Final concentration:"  -  empty space ("0" is not equal to "empty"; empty for unknown; “0” is value)
"Volume of the solvent:" - 100,
"Volume of the stock:" - 130
and  press "Calculate".

How is the proportion calculated ("the rules of the cross")

Determine the mixing ratios of two substances online with the dilution calculator. It is used to simplify the calculations in the case of a solution given concentration (in mass fraction,%) by diluting the solvent or mixture of two fluids or a mixture of two solutions with different pH. The desired concentration of the solution is written at the intersection of two lines, and the concentration of initial solutions (for the solvent, it is equal to zero) - in both ends of the lines on the left. Then, for each line, the computation of one standing on it number from another, and the difference is recorded at the free end of the same line. Calculations indicate the direction of the arrows. The resulting number is appropriate to have at the ends of the line on the right. They show how many units of the mass of each solution should be taken to obtain a solution with this given concentration.