$T=\sqrt{\frac{2h}{g}}\left[\frac{1+e}{1-e}\right]$

               $4=\sqrt{\frac{2\times 20}{10}}\left(\frac{1+e}{1-e}\right)$

               $4=2\left(\frac{1+e}{1-e}\right)$

               $4-4e=2+2e$

               $6e=2\Rightarrow e=\frac{2}{6}=\frac{1}{3}=0.3$

               $h^1=h\left[\frac{1+e^2}{1-e^2}\right]=20\left[\frac{1+0.09}{1-0.09}\right]=23.95m$