Find the total mass to be supported
hint-- it's the mass of the yogurt tin and the mass of the empty balloon.
Find the mass of the helium balloon--
hint its the volume of the balloon in m^3 times the density of helium
Find the mass of the displaced air
hint it's the volume of the balloon in m^3 time the density of the cabin air
The mass of the tin of yogurt and the empty balloon is offset by the mass difference of the helium and the cabin air. Set these equal and solve for volume.
hint-- it's in m^3