The solution contains 30% salt. If you add 100 g of salt, then the solution will contain 80% salt.

The solution contains 30% salt. If you add 100 g of salt, then the solution will contain 80% salt. How many grams of salt was in the solution initially?

Let’s designate the initial mass of the solution as “x” gram, then the mass of salt in this solution will be equal to “0.3x” gram.
After adding 100 grams of salt to the solution, its mass became equal to “x + 100” grams, and the new mass of salt was equal to “0.8 * (x + 100)” or “0.3x + 100”.
Let’s make an equation.
0.8 * (x + 100) = 0.3x + 100,
0.8x + 80 = 0.3x + 100,
0.5x = 20,
x = 20 / 0.5 = 40 grams.
That is, the initial mass of the solution was 40 grams.
40 * 30% = 40 * 0.3 = 12 grams of salt was originally in the solution.