How do I calculate the final concentration of a solution with different concentrations?

The final amount of solute and the final amount of the whole solution are both required in order to determine the final concentration. This can be calculated by determining the amount of solute in both original mixtures, adding them together, and then dividing by the total amount of solution. To be written as a percent concentration, the final concentration can be multiplied by 100. 

As an example, one solution has a 3% solution (by mass) of salt in water, with 80 g of solution in total, and another solution has a 9% solution of salt in water with 130 g of solution in total. The first solution contains 0.03 x 80g = 2.4 g of salt, and the second solution contains 0.09 x 130 g = 11.7 g of salt. The sum of the two is 2.4 + 11.7 = 14.1 g of salt and 80 g + 130 g = 210 g of solution. This results in a 14.1 g / 210 g = 0.067 or 6.7% concentration final solution.