Answer:
a. The time required for the tank to empty halfway is presented as follows;

b. The time it takes for the tank to empty the remaining half is presented as follows;

The total time 't', is presented as follows;

Explanation:
a. The diameter of the tank = D₀
The height of the tank = H
The diameter of the orifice at the bottom = D
The equation for the flow through an orifice is given as follows;
v = √(2·g·h)
Therefore, we have;


Where;
P₁ = P₂ = The atmospheric pressure
z₁ - z₂ = dh (The height of eater in the tank)
A₁·v₁ = A₂·v₂
v₂ = (A₁/A₂)·v₁
A₁ = π·D₀²/4
A₂ = π·D²/4
A₁/A₂ = D₀²/(D²) = v₂/v₁
v₂ = (D₀²/(D²))·v₁ = √(2·g·h)
The time, 'dt', it takes for the water to drop by a level, dh, is given as follows;
dt = dh/v₁ = (v₂/v₁)/v₂·dh = (D₀²/(D²))/v₂·dh = (D₀²/(D²))/√(2·g·h)·dh
We have;

The time for the tank to drop halfway is given as follows;

![t_1 =\left[{ \dfrac{D_0^2}{D\cdot \sqrt{2\cdot g} } \cdot\dfrac{h^{-\frac{1}{2} +1}}{-\frac{1}{2} +1 } \right]_{\frac{H}{2} }^{H} =\left[ { \dfrac{D_0^2 \cdot 2\cdot \sqrt{h} }{D\cdot \sqrt{2\cdot g} } \right]_{\frac{H}{2} }^{H} = { \dfrac{2 \cdot D_0^2 }{D\cdot \sqrt{2\cdot g} } \cdot \left(\sqrt{H} - \sqrt{\dfrac{H}{2} } \right)](https://tex.z-dn.net/?f=t_1%20%20%3D%5Cleft%5B%7B%20%5Cdfrac%7BD_0%5E2%7D%7BD%5Ccdot%20%5Csqrt%7B2%5Ccdot%20g%7D%20%7D%20%5Ccdot%5Cdfrac%7Bh%5E%7B-%5Cfrac%7B1%7D%7B2%7D%20%2B1%7D%7D%7B-%5Cfrac%7B1%7D%7B2%7D%20%2B1%20%7D%20%5Cright%5D_%7B%5Cfrac%7BH%7D%7B2%7D%20%7D%5E%7BH%7D%20%3D%5Cleft%5B%20%7B%20%5Cdfrac%7BD_0%5E2%20%5Ccdot%202%5Ccdot%20%5Csqrt%7Bh%7D%20%7D%7BD%5Ccdot%20%5Csqrt%7B2%5Ccdot%20g%7D%20%7D%20%5Cright%5D_%7B%5Cfrac%7BH%7D%7B2%7D%20%7D%5E%7BH%7D%20%3D%20%7B%20%5Cdfrac%7B2%20%5Ccdot%20D_0%5E2%20%7D%7BD%5Ccdot%20%5Csqrt%7B2%5Ccdot%20g%7D%20%7D%20%5Ccdot%20%5Cleft%28%5Csqrt%7BH%7D%20-%20%5Csqrt%7B%5Cdfrac%7BH%7D%7B2%7D%20%7D%20%5Cright%29)

The time required for the tank to empty halfway, t₁, is given as follows;

(b) The time it takes for the tank to empty completely, t₂, is given as follows;

![t_2 =\left[{ \dfrac{D_0^2}{D\cdot \sqrt{2\cdot g} } \cdot\dfrac{h^{-\frac{1}{2} +1}}{-\frac{1}{2} +1 } \right]_{0}^{\frac{H}{2} } =\left[ { \dfrac{D_0^2 \cdot 2\cdot \sqrt{h} }{D\cdot \sqrt{2\cdot g} } \right]_{0 }^{\frac{H}{2} } = { \dfrac{2 \cdot D_0^2 }{D\cdot \sqrt{2\cdot g} } \cdot \left( \sqrt{\dfrac{H}{2} } -0\right)](https://tex.z-dn.net/?f=t_2%20%20%3D%5Cleft%5B%7B%20%5Cdfrac%7BD_0%5E2%7D%7BD%5Ccdot%20%5Csqrt%7B2%5Ccdot%20g%7D%20%7D%20%5Ccdot%5Cdfrac%7Bh%5E%7B-%5Cfrac%7B1%7D%7B2%7D%20%2B1%7D%7D%7B-%5Cfrac%7B1%7D%7B2%7D%20%2B1%20%7D%20%5Cright%5D_%7B0%7D%5E%7B%5Cfrac%7BH%7D%7B2%7D%20%7D%20%3D%5Cleft%5B%20%7B%20%5Cdfrac%7BD_0%5E2%20%5Ccdot%202%5Ccdot%20%5Csqrt%7Bh%7D%20%7D%7BD%5Ccdot%20%5Csqrt%7B2%5Ccdot%20g%7D%20%7D%20%5Cright%5D_%7B0%20%7D%5E%7B%5Cfrac%7BH%7D%7B2%7D%20%7D%20%3D%20%7B%20%5Cdfrac%7B2%20%5Ccdot%20D_0%5E2%20%7D%7BD%5Ccdot%20%5Csqrt%7B2%5Ccdot%20g%7D%20%7D%20%5Ccdot%20%5Cleft%28%20%5Csqrt%7B%5Cdfrac%7BH%7D%7B2%7D%20%7D%20-0%5Cright%29)

The time it takes for the tank to empty the remaining half, t₂, is presented as follows;

The total time, t, to empty the tank is given as follows;

