Final answer:
The weight of the brass cylinder in water is 80 gm.
Step-by-step explanation:
To calculate the weight of the brass cylinder in water, we can use Archimedes' principle which states that the buoyant force on an object submerged in a fluid is equal to the weight of the fluid displaced by the object.
Since the density of brass is given as 9 gm/cc, we can calculate the volume of the cylinder by dividing its mass (90 gm) by its density. This gives us a volume of 10 cc.
Now, when the cylinder is submerged in water, it will displace 10 cc of water. Since the density of water is 1 gm/cc, the weight of the water displaced is equal to its mass, which is 10 gm.
Therefore, the weight of the cylinder in water is 90 gm - 10 gm = 80 gm.