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
[tex]V = \frac{4}{3}\pi r^3[/tex]
here radius R = 16 cm
[tex]V = \frac{4}{3}*\pi*0.16^3[/tex]
V = 0.0172 m^3
Now the buoyancy force of air on each balloon is given by
[tex]F_b = \rho * V * g[/tex]
[tex]F_b = 1.2 * 0.0172 * 9.8[/tex]
[tex]F_b = 0.202 N[/tex]
Now weight of the person that we have to counter balance
[tex]W = 75*9.8 = 735 N[/tex]
Now by force balance
[tex] N*F_b = W[/tex]
[tex] N*0.202 = 735[/tex]
[tex] N = 3639[/tex]
So we require 3639 balloons to hold the person of mass 75 Kg