we do
concentration=amount times percent
total concentration=concentration8+concenctration20
so
12 times 8%+x times 20%=16% times (12+x)
x=amount of 20% acis solution
120.08+0.2x=0.16*(12+x)
solve
distribute and simplify
0.96+0.2x=1.92+0.16x
minus 0.96 from both sides
0.20x=0.96+0.16x
minus 0.16x from both sides
0.04x=0.96
divide both sides by 0.04
x=24
24 liters of the 20% was added