Question 73485
Let x = the required number of liters of 10% acid solution, then (30-x) = the required number of liters of 25% acid solution. The sum of these will make up the 30 liters of 18% acid solution. After changing the percentages to their decimal equivalents, we can write the necessary equation to solve for x.

{{{0.1x + 0.25(30-x) = 0.18(30)}}} Simplify and solve for x.
{{{0.1x + 7.5 - 0.25x = 5.4}}} Combine like-terms.
{{{-0.15x + 7.5 = 5.4}}} Subtract 7.5 from both sides.
{{{-0.15x = -2.1}}} Divide both sides by -0.15
{{{x = 14}}} and {{{30-x = 30-14}}} = {{{16}}}
Solution:
You will need 14 liters of 10% acid solution plus 16 liters of 25% acid solution to make 30 liters of 18% acid solutution.