Question 13778
First, change the percents to decimals:  18% = 0.18, 50% = 0.5, 30% = 0.3
Now set up the necessary equation. Let x = the number of liters of 18% alcohol solution and (80 - x) = the number of liters of 50% alcohol solution.

{{{(0.18)x + (0.5)(80 - x) = (0.3)(80)}}} Simplify and solve for x.
{{{0.18x - 0.5x + 40 = 24}}} Subtract 40 from both sides.
{{{-0.32x = -16}}} Divide both sides by -0.32
{{{x = 50}}}

The chemist will need 50 liters of the 18% alcohol solution and (80-50=30) 30 liters of the 50% alcohol solution to make 80 liters of 30% alcohol solution.