$\quad\quad\quad50\, {ml}, 1\, {M} \quad 30\, {ml}, 1\, {M} \quad\quad\quad\quad-\quad\quad\quad\quad\quad-$
${t}=0 \,\quad 50 \,{~mm} \,\quad\quad 30\, {~mm}$
${t}=\infty\quad 20\, {~mm} \quad\quad\quad\, -$
$[{HCl}]=\frac{20}{80}=\frac{1}{4}\, {M}=2.5 \times 10^{-1}\, {M}$
${pH}=-\log 2.15 \times 10^{-1}=1-0.3979=0.6021$
${pH}=6021 \times 10^{-4}$