A body weighs 40 g in air. If it's volume is 10 cc, find its weight in water.

Loss of weight in water is equal to buoyant force.

$$\begin{gathered} Loss of weight=Buoyant force \\ =weight of the water displaced by immersed volume \\ =\rho Vg \\ =1\times 10\times g \\ =10g \end{gathered}$$

Weight in water:

Weight in water = Weight in air - Loss of weight in water
                         = 40 g - 10 g
                        = 30 g