We know that the acid solution is 600 mL, 75% acid. We need to get a solution with 25% acid.
Based on the given information, we express the following
[tex]0.75\cdot600+0x=0.25(600+x)[/tex]Where x represents water. Let's solve for x
[tex]\begin{gathered} 450=150+0.25x \\ 450-150=0.25x \\ 0.25x=300 \\ x=\frac{300}{0.25} \\ x=1,200 \end{gathered}[/tex]