Final answer:
The upthrust of water on an object, according to Archimedes' principle, is equal to the weight of the water displaced by the object. In this scenario, since the weight of the water is 15 Newtons, the upthrust is also 15 Newtons.
Step-by-step explanation:
The question involves the upthrust of water, which refers to the buoyant force exerted on an object submerged in a fluid. By Archimedes' principle, the upthrust (buoyant force) on an object in a fluid is equal to the weight of the fluid it displaces. In the case where the weight of the water is given as 15 Newtons, this weight itself represents the upthrust, as the object would displace an amount of water equal to its own weight in order to float.
Therefore, if an object is completely submerged and is in a state of equilibrium (not sinking or rising), the upthrust on it by the water will be equal to the weight of the object. Since the weight of the water displaced by the object is provided as 15 Newtons, the upthrust on the object is also 15 Newtons.