$PH=3/7\Rightarrow \left[ {{H}^{+}} \right]={{10}^{-3/7}}={{10}^{-4}}\times {{10}^{0/3}}=2\times {{10}^{-4}}$  $PH=5/3\Rightarrow \left[ {{H}^{+}} \right]={{10}^{-5/3}}={{10}^{-6}}\times {{10}^{0/7}}=5\times {{10}^{-6}}$  $\frac{2\times {{10}^{-4}}}{5\times {{10}^{-6}}}=\frac{200}{5}=40$