Answer
$D=0.184\ m$
Work Step by Step
Given the volume:
$V=6\times5\times4=120\ m³$
And the emptying time of 20 min:
$\dot{V}=V/\Delta t$
$\dot{V}=0.1333\ m³/s$
Since: $\dot{V}=A_c\mathcal{V},\ A_c=\dfrac{\pi D^2}4,\ \mathcal{V}=5\ m/s$
solving for the diameter:
$D=0.184\ m$
