How much salt should be added to 500 g of a 10% salt solution to make the salt solution 25%?

From this task it follows that it is necessary to find the mass of salt, which must be added to a 10% solution.
In order to solve this problem, you first need to find the mass of salt in a 10% solution.
For this we use the following formula
W = m r.v. / m p-pa × 100%
We substitute the data and we get the following:
0.1 = x / 500
We find x from the resulting equation and we get the following solution:
x = 0.1 × 500
x = 50 g
It turns out that a 10% solution contains 50 grams of salt.
Next, we compose the equation
0.25 = (50 + x) ÷ (500 + x)
50 + x = 125 + 0.25 x
0.75 x = 75
x = 100 g
Answer: 100 grams of salt must be added to a 10% solution to make it a 25% solution.