Filling the tank

According to the question, the volume to be filled is equivalent to the volume of a cuboid of dimensions $50\text m\times 44\text m\times 21\text{cm}$ .

Which is equal to $50\times 44\times \dfrac{21}{100}=462\text m^3$ .

We have the speed of water to be $15 \text{km/hr} = 15000 \text{m/hr}$ , and the radius of the pipe is $7 \text{cm} = 0.07 \text{m}$ . We can find the volume of water flowing into the tank per hour, which is: $\pi r^2\times \text{speed of water} = \dfrac{22}{7}\times 0.07^2\times 15000 = 231\text{m}^3\text{/hr}$

So, the time taken by the pipe to fill the water is $=\dfrac{462}{231}=\boxed{2\text{hr}}=\boxed{120\text{min}}$ .