Question

a stone weights 25 Newton in air and 12 Newton when immersed in water the magnitude of buoyant force acting on the stone in water is

Answer 1

Weight of a stone in water = 25 N

Weight of the stone when it is immersed in water = 12 N

So, 

Magnitude of buoyant force acting on the stone

= (25 - 12) N

= 13 N

Answer 2

Buoyancy or buoyant force, is an upward force presented by a liquid that opposes the weight of an object inside it or the downward force acting on it.

Weight of a stone in water = 25 N

Weight of the stone when it is immersed in water = 12 N

So,

Magnitude of buoyant force acting on the stone

= (25 - 12) N

= 13 N