## Elementary Algebra

Let X represent the amount of acid to be added in the 100ml solution. After this amount is added, we obtain a solution with a 20% acid concentration. We use the following guideline to solve this problem: Amount of pure acid in original solution + Amount of pure acid to be added = Amount of pure acid in final solution. So: 10% $\times$ 100ml + X = 20% $\times$ (100+X) .1 x 100mL+X= .2 x (100+X) 10 + X = 20 + $\frac{X}{5}$ 10 = X - $\frac{X}{5}$ 10 = $\frac{4X}{5}$ Multiply both sides by 5 50 = 4X Divide both sides by 4 X = 12.5 ml 12.5 ml of acid must be added in the solution.