In given case mass of rocket is changing with time, but velocity of gases remained constant.
Therefore Thrust=v (dm/dt) -eq(1)
Where dm/dt is rate at which mass of gas is ejected.
Resultant acceleration = Acceleration due to thrust (up) - acceleration due to gravity(down)
20 = a - 10 Or a = 30 is the acceleration produced by thrust
According newton's second law of thrust=(mass)×(acceleration)...(2)
From equation (1) and (2) we get
ma=v ( dm/dt)
5000 × 30=800 ( dm/dt ) On solving equation for (dm/dt)we get