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"A lighter than air balloon and its load of passengers and ballast are floating stationary above the Earth. Ballast is the weight that van be dropped over board to make the balloon rise. The radius of the balloon is 6.25m. Assuming the density of air to be 1.29kg/m^3, determine the weight that must be dropped overboard to make the balloon rise 105m in 15 secs."

so far i've determined the balloon has to rise at a rate of 7m/s to reach 105m in 15seconds. I was then going to use the Buoyancy formula to figure out how much force is pushing up on the balloon, but i don't understand how to do that because i don't have the volume of the balloon..