d
\(H = \frac{{{V^2}t}}{R}.\)

\(V^2t =\) constant \( \Rightarrow \)  \(t \propto \frac{1}{{{V^2}}}\) 

\(\frac{{{t_1}}}{{{t_2}}} = {\left( {\frac{{{V_2}}}{{{V_1}}}} \right)^2} \Rightarrow \frac{5}{{{t_2}}} = {\left( {\frac{{110}}{{220}}} \right)^2} = \frac{1}{4}\, \Rightarrow \,{t_2} = 20\,\min \)