the weight of a piece of stone when fully immersed in water is 18N and it displaces 4N of water. what is the weight of the stone in air?

4N

Explanation:

From the problem statement, the weight of the piece of stone when fully immersed in water is 18N

The stone displaces water with the weight of 4N;

 So, the weight of the stone in air is 4N.

From Archimedes principle, we know that the weight of the water displaced when an object is placed in a liquid is the weight of the object. This method is often used to determine the weight of objects that are irregularly shaped. The weight of a regular body  can be easily determined. But for irregular bodies, immersion in a liquid is a good approximation.

Answeraquifer;