Prior to its launch, a balloon carrying a set of experiments to high altitude is held in place by groups of student volunteers holding the tethers at b, c, and d. the mass of the balloon, experiments package, and the gas it contains is 90 kg, and the buoyancy force on the balloon is 1000 n. the supervising professor conservatively estimates that each student can exert at least a 40-n tension on the tether for the necessary length of time. based on this estimate, what minimum numbers of students are needed at b, c, and d?