b
(b)Time taken to be emptied for \(h\) height, \(t = \sqrt {\frac{{2h}}{g}} \) and for \(\frac{h}{2}\) height, \(t' = \sqrt {\frac{{2h/2}}{g}} = \sqrt {\frac{h}{g}} \)\(\frac{{t'}}{t} = \frac{1}{{\sqrt 2 }} \Rightarrow t' = \frac{t}{{\sqrt 2 }} = \frac{{10}}{{\sqrt 2 }} = 7\;minute\)