$A$ vessel filled with water is kept on a weighing pan and the scale is adjusted to zero. $A$ block of mass $M$ and density $\rho$ is suspended by a massless spring of spring constant $k$. This block is submerged into the water in the vessel. What is the reading of the scale?

(N/A) Consider the system consisting of the vessel and the water. The scale is initially adjusted to zero.
When the block is submerged in the water,it experiences an upward buoyant force (upthrust) $F_B$ from the water.
According to Newton's third law,the block exerts an equal and opposite downward force on the water.
Therefore,the reading of the scale increases by an amount equal to the upthrust experienced by the block.
Upthrust $F_B = V_{sub} \rho_w g$,where $V_{sub}$ is the submerged volume of the block and $\rho_w$ is the density of water.
If the entire block of mass $M$ and density $\rho$ is submerged,then $V_{sub} = V = \frac{M}{\rho}$.
Thus,the reading of the scale is $F_B = \left( \frac{M}{\rho} \right) \rho_w g = M g \left( \frac{\rho_w}{\rho} \right)$.