An urgent question about physical buoyancy

A hollow iron ball with a mass of 79 g is suspended when immersed in water. Find the volume of the hollow part of the iron ball. (The density of iron is 7.9×l03kg/m3)

19 A glass ball with a mass of 200g and a volume of 250 cm3 is thrown into the water. Find (1) the gravity force on the glass ball; (2) the buoyancy force on the glass ball when it is immersed in water; (3) the buoyancy force on the glass ball when it is at rest.

7 The helium balloon bag together with the slogan strip it carries weighs 200N and has a volume of 30 m3. Assume that the air tightness becomes l.3 ×l03kg/m3. Question: (1) How much buoyant force do helium balloons and banners receive in the air? (2) If the balloon and the banner are to be suspended in the air, what is the minimum tensile force on the string holding the balloon on the ground? (The density of helium is 0.18kg/m3)

19 A glass ball with a mass of 200 g and a volume of 250 cm3 is thrown into the water. Find (1) the gravity force on the glass ball; (2) the buoyancy force on the glass ball when it is immersed in water; (3) the buoyancy force on the glass ball when it is at rest.

18 Archaeologists used air bags to salvage stone cultural relics with a mass of 1,000kg from the water. The stone cultural relics are pulled upward by the air bag in the water with a force of 6000N, and float up at a constant speed. Find: (1) the buoyancy force experienced by stone cultural relics in water; (2) the density of stone cultural relics.