C: 10 L of the 20% solution
Let
x = amount of 8% solution
y = amount of 20% solution
The chemist wants to end up with a 12-L mixture, so
x + y = 12
x L of 8% solution contains 0.08x L of salt, and y L of 20% solution contains 0.2y L of salt. When combined, the resulting mixture should have a concentration of 18% salt, or 18% of 12 L = 2.16 L of salt, so
0.08x + 0.2y = 2.16
Solve for y :
x = 12 - y
0.08 (12 - y) + 0.2y = 2.16
0.96 - 0.08y + 0.2y = 2.16
0.12y = 1.2
y = 10