Two solutions of salt water contain 0.04% and 0.2% salt respectively. A lab technician wants to make 1 liter of solution which contains 0.12% salt. How much of each solution should she use?

x = amount (in L) of 0.04% solution

y = amount (in L) of 0.2% solution

x + y = 1

Each liter of p% salt solution contributes 0.01*p L of salt to the mixture. In the new solution, the lab tech wants to end up with a concentration of 0.12%, which comes out to 0.0012 * (1 L) = 0.0012 L of salt:

0.0004x + 0.002y = 0.0012

Solve for y in the first equation:

y = 1 - x

Substitute this into the other equation and solve for x, then y:

0.0004x + 0.002 (1 - x) = 0.0012

0.0008 = 0.0016x

x = 0.5 L

y = 1 - 0.5 = 0.5 L