\(\frac{-H}{2}=u t-\frac{1}{2} g \times 10^2\)

\(\Rightarrow H=g \times 10^2\)

\(\Rightarrow -H=-\frac{1}{2} g t^2 \quad\) (Full joumey)

\(g \times 10^2=\frac{1}{2} g t^2\)

\(\Rightarrow t^2=200\)

\(\Rightarrow t=10 \sqrt{2} s\)

\(\Rightarrow t=10 \times 1.414 s\)

\(=14.14 s =t\)