The volumes of 1M HCl and 5M HCl to be mixed to get 2 lit of 2M HCl are

Total volume of the mixture = 2l , Molarity of the mixture = 2M

  sample(I)                    sample (II)

V₁ = xlit                       V₂ = (2-x) lit

M₁ = 1M                       M₂ = 5M

Resultant Molarity 

, $$\begin{gathered} \mathrm{M}=\frac{{\mathrm{V}}_1{\mathrm{M}}_1+{\mathrm{V}}_2{\mathrm{M}}_2}{{\mathrm{V}}_1+{\mathrm{V}}_2} \\ 2 =\frac{\mathrm{x}\left(1\right)+(2-\mathrm{x})5}{2} \end{gathered}$$

4=x+10_5x                     

 4x=6                          

x=1.5l

                     volume of 1M HCl required = 1.5l

    volume of 5M HCl required  = (2-1.5)l = 0.5l