The number of water molecules in gypsum and plaster of Paris, respectively x and y. The ratio of y and x is                  .

 

Gypsum: ${\mathrm{CaSO}}_4·2{\mathrm{H}}_2\mathrm{O}$

So, the value of $x$ is 2

Plaster of Paris: ${\mathrm{CaSO}}_4·\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$

So, the value of $y$ is $\frac{1}{2}$.

Ratio of $y$ and 

$$\begin{gathered} x=\frac{1}{2}÷2 \\ =0.25 \end{gathered}$$

Hence, The ratio of $y$ and $x$ is $0.25$.