A student wants to find the value of $x$ in the formula of hydrated copper(II) sulfate, ${CuSO}_4\cdot x{H}_{2}O$. They weigh a sample of the hydrated crystals, heat them in a crucible to drive off the water of crystallisation, and then reweigh the remaining anhydrous copper(II) sulfate until a constant mass is reached. The equation representing the chemical change is:

$${CuSO}_4\cdot x{H}_{2}O\to {CuSO}_4+x{H}_{2}O$$

These are the student's results:

• mass of dry hydrated crystals before heating = $12.5g$
• mass of anhydrous crystals after heating to a constant mass = $8.0g$ Use these results to find the value of $x$ in the formula of ${CuSO}_4\cdot x{H}_{2}O$. [$M_r$ values: ${CuSO}_4=160$, $H_{2}O=18$]