Answer:
100 grams of salt.
Explanation:
First of all, we'll determine our variables:
let x=grams of salt that you already have
let y=grams of salt that need to be added
let z=total amount of salt in the solution after adding salt
We know that salt makes up 25% of the 400g solution, and that 25% can also be written as .25. Thus, we can write an equation to determine x's value:
x=.25*400g
x=100g
Therefore, there are 100 grams of salt already in the solution.
Now that we've determined x's value, we'll bring z into the equation. There are two ways to calculate z. z is equivalent to the sum of x and y. Remember, x is equal to 100 grams. So, z's equation is as follows:
z=100g+y
The other way to calculate z is as a percentage of the final solution. As the question states, we want the final solution to have a salt content of 40% (which we can write as .40). The final amount of solution can be written as below:
400g+y=total amount of solution after adding salt.
This is because we are adding y (the amount of salt needed to bring the salt content to 40 percent)to the amount of solution which we already have.
Therefore, the second way to express z is below:
z=.40*(400g+y)
After we have two equations for z, we can set them equal to each other as follows:
100g+y=.40*(400g+y)
We then can distribute the .40 over the parenthesis.
100g+y=160g+.40y
And then we can subtract 100 grams from both sides of the equation.
y=60g+.40y
Next, we subtract .40 y from both sides of the equation to get y on one side.
.6y=60g
Finally, we can divide both sides by .6 to isolate y.
y=100g
Therefore, you must add 100g of salt to the solution to make it 40 percent salt.