a 500kg is into a vertical firing.If the exhaust speed of gas is 800m/s to attain 20m/s upward acceleration and overcomes gravitational pull,.the amount of gas ejected per second?

Since it overcome the gravitational pull,and since there is only one external force acting on the system therefore f(ext)=0.We also know that f(ext)+f(th)=Ma,where f(th) is the upthrust produced .therefore applying the values we get 500*20=0+dm/dt(800)=dm/dt=10000/800=12.5 kg/s.