Determine whether the tank will overflow with mass flowrate

Hydrocarbon mixture ($\pu{20 mol\%}$ $\ce{CH4}$ and the balance is $\ce{C2H6}$) enters a tank at a velocity of $\pu{1 m/s}$ and is withdrawn at a rate of $\pu{2 kg/s}$. The tank is initially one quarter full. The diameter of the pipe is given as $\pu{15 cm}$ and the height of the tank is $\pu{2 m}$. Density of $\ce{CH4}$ is $\pu{422.36 kg/m^3}$ and density of $\ce{C2H6}$ is $\pu{543.83 kg/m^3}$. The volumetric flowrate of the hydrogen mixture entering the tank is $\pu{6361.56 m^3/h}$ 


Determine the mass flowrate ($\pu{kg/h}$) of hydrocarbon entering the tank and determine how long ($\pu{s}$) does the tank take to overflow or to be emptied. 


As for determining the mass flow rate, of the inlet, I used the densities of the component to help me get the total mass flowrate. This is what I have done: 


$$\text{Density} = \frac{\text{Mass flowrate}}{\text{volumetric flowrate}}$$ 


For $\ce{CH4}$: 
$$\text{Mass flowrate} = (422.36)(6361) = \pu{2686868.482 kg/h}$$ 


For $\ce{C2H6}$: 
$$\text{Mass flowrate} = (543.83)(6361) = \pu{3459302.63 kg/h}$$ 


Therefore, total mass flowrate into the tank is $\pu{6146171.112 kg/h}$. 


Now I'm stuck on the question to determine how long does it take for the tank to overflow or be emptied.  

Should I use the mass flowrate to determine or the volumetric flowrate? 
I assumed is the mass flowrate, but I am unsure how to start. 


