Several people are riding in a hot air balloon. The combined mass of the balloon and the people (from here on after to be called the system) totals 355 kg. The system is initially motionless in the air because its downward-acting weight is balanced by an upward-acting buoyant force produced by the surrounding air. If the buoyant force remains constant how much mass should be dropped overboard so that the system acquires a net upward acceleration of .150 m/s2? (Be sure to take notes of positive and negative values in this problem)