The third and fourth stages of a rocket are coasting in space with a velocity of 15,000 km/h when the fourth-stage engine ignites, causing seperation of the stages under a thrust T and its reaction. If the velocity v of the fourth stage is 10 m/s greater than the velocity v' of the third stage at the end of the 1/2 s seperation interval (half a second), calculate the average thrust T during this period. The masses of the third and fourth stages at seperation are 40 kg and 60 kg, respectively. Assume that the entire blast of the fourth-stage engine impinges against the third stage with a force equal and opposite to T.