Archimedes' principle says that a 15 N object is buoyed up by a force that is

equal to the weight of the fluid that the body displaces

Explanation:

Archimides' principle states the definition of buoyant force.

These are the imporant features of the buoyant force that you need to know:

Except for the very special vacuum condition, every body is immersed in a fluid.The body is subject, at least, to two forces, its weight and the buoyant force.The weight is the result of the gravitational attraction exerted by the planet, and is equal to the mass of the object by the gravitational acceleration (approximately 9.8 m/s² on Earth). It is directed vertically downward.The buoyant force is the vertical upward force that the fluid exerts on the object and is equal to the weight of the volume of fluid displaced by the object.The volume of fluid that the body displaces is equal to the immersed volume of the body.