To 500 g of a 12% salt solution was added 300 g of an 8% salt solution. What is the concentration of the resulting mixture?

500 grams of 12% salt solution contains salts:

500 * 12: 100 = 60 (g).

300 grams of 8% salt solution contains salts:

300 * 8: 100 = 24 (g).

Therefore, when mixing these solutions, the mass of salt in the resulting solution will be equal to:

60 + 24 = 84 (g).

Since the mass of the resulting solution is

500 + 300 = 800 (g),

then the concentration of salt in it is equal to:

84: 800 * 100 = 10.5%.

Answer: 10.5%.