A hemispherical tank full of water …

We have, Radius of hemispherical tank = {tex} \frac { 3 } { 2 } m{/tex}
{tex} \therefore{/tex} Volume of the tank ={tex} \frac { 2 } { 3 } \times \frac { 22 } { 7 } \times \left( \frac { 3 } { 2 } \right) ^ { 3 } \mathrm { m } ^ { 3 } = \frac { 99 } { 14 } \mathrm { m } ^ { 3 }{/tex}
Volume of the water to be emptied = {tex} \frac { 1 } { 2 } \times \frac { 99 } { 14 } \mathrm { m } ^ { 3 } = \frac { 99 } { 28 } \mathrm { m } ^ { 3 } = \frac { 99 } { 28 } \times 1000 \text { litres } = \frac { 99000 } { 28 }{/tex}litres
Since {tex} \frac { 25 } { 7 }{/tex} litres of water is emptied in one second. Therefore,
Total time taken to empty half the tank i.e {tex} \frac { 99000 } { 28 }{/tex} litres of water = {tex} = \frac { 99000 } { 28 } \div \frac { 25 } { 7 }{/tex}seconds
{tex} = \frac { 99000 } { 28 } \times \frac { 7 } { 25 }{/tex}seconds
{tex} = \frac { 99000 } { 28 } \times \frac { 7 } { 25 } \times \frac { 1 } { 60 }{/tex}minutes
= 16.5 minutes