How many grams of salt should be added to 200 g of a 10% salt solution to get a 20% solution.

Given:
Initial solution: concentration 10% = 0.1; weight 200 g;
Necessary solution: concentration 20% = 0.2;

Decision:
Initially in a salt solution: 0.1 * 200 = 20 g; liquids: 200-20 = 180 g;
Concentration calculation = weight of salt / (weight of salt + weight of liquid);
Additional amount of salt y g;

0.2 = (20 + y) / (200 + y);
0.2 * (200 + y) = 20 + y;
40 + 0.2 * y = 20 + y;
40-20 = y-0.2 * y;
20 = 0.8 * y;
y = 20 / 0.8;
y = 25;

Answer: you need to add 25 g of salt.