Answer
$m_2=411.5\ kg$
Work Step by Step
The material balance for the tank:
$\Delta m = (\dot{m}_{in}-\dot{m}_{out})\Delta t$
Constant density:
$\Delta V = (\dot{V}_{in}-\dot{V}_{out})\Delta t$
With $V_1=0.3\ m³,\ \dot{V}_{in}=15L/min=0.00025m³/s$,
$\dot{V}_{out}=\frac{\pi}{4}D^2\nu_{out},\ D=0.02m, \nu_{out}=0.5\ m/s, \Delta t=20 min$:
$V_2=0.4115\ m³$
With $\rho=1000\ kg/m³$:
$m_2=411.5\ kg$
