A sphere of iron and another of wood of the same radius are held under water. Compare the upthrust on the two spheres. [Hint — Both have equal volume inside water.]

Let density of water be ρ_w

Since radius of both spheres are same hence their volume will be equal.
Let volume of iron sphere = volume of wood sphere = V

Upthrust on Iron sphere = Upthrust_iron = Vρ_wg

Upthrust on Wood sphere = Upthrust_wood = Vρ_wg

Comparing the upthrust on the two spheres:

Upthrust_iron : Upthrust_wood = Vρ_wg : Vρ_wg = 1 : 1