How many water molecules are in a $0.10 g$ sample of ${\mathrm{CuSO}}_4\cdot 5{\mathrm{H}}_2\mathrm{O}(\mathrm{MM}=249.7)$?

There are $1.2\times {10}^{21}$ water molecules are in a $0.10 g$ sample of ${\mathrm{CuSO}}_4\cdot 5{\mathrm{H}}_2\mathrm{O}(\mathrm{MM}=249.7)$

$$\begin{gathered} N=\frac{0.10}{249.7}\times 5\times \left(6\times {10}^{23}\right) \\ N=1.2x{10}^{21} \end{gathered}$$