m1*v1+m2*v2=(m1+m2)*V
V=(m1*v1+m2*v2)/(m1+m2)
V=(20kg*500m/c+10000kg*0m/c)/(20+10000)=10000/100200=0.99m/c
m1*v1+m2*v2=(m1+m2)*V
V=(m1*v1+m2*v2)/(m1+m2)
V=(20kg*500m/c+10000kg*0m/c)/(20+10000)=10000/100200=0.99m/c