Suppose that a party balloon is spherical, has a radius of 16 centimetres and is filled with helium. furthermore suppose that we do our experiment under standard atmospheric conditions, when the air density is . how many of these balloons should a 75 kilogram human carry in order to become airborne?

Let N number of balloons are used to pull the person upwards

So there would be buoyancy force due to air on balloons which will be counterbalanced by the weight of a person.

Volume of one balloon is given by

here radius R = 16 cm

V = 0.0172 m^3

Now the buoyancy force of air on each balloon is given by

Now weight of the person that we have to counter balance

Now by force balance

So we require 3639 balloons to hold the person of mass 75 Kg

i think the answer is a and b

i wish you luck