A boy stands on a stationary ice boat on a frictionless horizontal flat iceberg. The boy and the boat have a combined mass of $$M=60\ \text{kg}$$. Two balls of masses $$m_1=10\ \text{kg}$$ and $$m_2=20\ \text{kg}$$ are placed on the boat. In order to get the boat moving, the boy throws the balls backward horizontally either in succession or both together. In each case the balls are thrown backward with a certain speed $$v_{rel}=6\ \text{m s}^{-1}$$ relative to the boat just when the ball is being thrown. The resulting speed of the system of the boat and the boy is