Solution:

$$\eqalign{
& {\text{After removed 12L}} \cr
& \,\,\,\,\,\,\,{\text{Acid}}\,\,\,\,\,\,\,\,:\,\,\,\,\,\,\,{\text{Water}} \cr
& \Rightarrow 12 \times \frac{5}{8},\,12\, \times \frac{3}{8} \cr
& \Rightarrow - 7.5,\, - 4.5 \cr
& {\text{Acid}}\,\,\,\,\,\,\,\,\,\,{\text{Water}} \cr
& \,\,\,{\text{20L }}\,\,\,\,\,\,\,\,\,\,{\text{ 12L}} \cr
& - 7.5{\text{ }}\,\,\,\,{\text{ }} - 4.5 \cr
& {\text{ }}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{ }} + 7.5 \cr
& \overline {\,12.5\,\,\,\,\,:\,\,\,\,\,\,15\,\,} \cr
& \,\,\,\,\,\,5\,\,\,\,\,\,\,\,:\,\,\,\,\,\,\,\,6 \cr} $$