The volume of water that must be added to a mixture of 250ml  of 6M HCl and 750ml of 2M HCl to obtain 2.5 M solution

$$\begin{gathered} {\mathrm{M}}_{\mathrm{mix}}=\frac{{\mathrm{V}}_1{\mathrm{M}}_1+{\mathrm{V}}_2{\mathrm{M}}_2}{{\mathrm{V}}_1+{\mathrm{V}}_2+\mathrm{x}} \\ 2.5=\frac{(250\times +6)+(750\times 2)}{250+750+\mathrm{x}} \\ 2.5=\frac{1500+1500}{1000+\mathrm{x}} \\ 1000+\mathrm{x}=\frac{3000}{2.5} \\ 1000+\mathrm{x}=1200 \\ \mathrm{X}=200\mathrm{ml} \end{gathered}$$