Which of the following salts will NOT have $10$ molecules of water of crystallisation?

Let's examine the chemical formulas of the given salts:
* Glauber's salt: ${\text{Na}}_2{\text{SO}}_4\cdot 10{\text{H}}_2\text{O}$ ($10$ molecules)
* Washing soda: ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$ ($10$ molecules)
* Borax: ${\text{Na}}_2{\text{B}}_4{\text{O}}_7\cdot 10{\text{H}}_2\text{O}$ ($10$ molecules)
* Epsom salt: ${\text{MgSO}}_4\cdot 7{\text{H}}_2\text{O}$ ($7$ molecules)

Therefore, Epsom salt has $7$ molecules of water of crystallisation instead of $10$.