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First you need to figure how much helium is the balloons. Each balloon's lift is reduced by the weight of the helium inside.
Once you know how much each balloon weighs, subtract that from the lifting power of the displaced air, which is 1.225kg * .01716 * 9.8N/kg, since each balloon occupies .01716 m^3
Now you know how many kg each balloon can lift; divide that into 75kg.