a rocket problem

[vabamyyr]

First the rocket doesnt move. Its initial mass is 0,27 kg. Then the rocket starts to emit gases. During one second it emits 0,09kg gas with velocity in system of rocket of 300m/s. How much time does it takes for rocket to receive 40m/s velocity and what is the max velocity a rocket can get when there is alltogether
0,18 kg gas?(force of resistance of air is left out)

[Tide]

What have you tried so far?

Here's a start. The speed of the rocket as a function of mass is:

v = -v_e \ln \frac {M}{M_0}