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So that the weight of the water displaced by an object is less than the weight of the object, it will sink in the fluid. This is the reason, how large ship can float but a small needle does not float in the water.

Fb = ρgv

And Fb = Wa – Wf

Therefore,

ρgv = Wa – Wf

Where,

Fb = buoyant force

ρ = density of fluid

g = gravitational acceleration

v = volume of object

Wa = Weight of object in air

Wf = Weight of object in fluid.

Weight of the air displaced by the balloon = Volume x density

= 4 x 1.294 = 5.176 g

Therefore, The true weight = 1.867 + 5.176 = 7.043 gm

mass of substance = 80 gm

Volume of substance = 30 cm^3

So that, we can calculate the density of the substance.

Density = mass/volume = 80/30 = 2.67 gm/cm^3

As the density of substance is more than the density of water, therefore it will sink into the water.