5 kg of anhydrous $\displaystyle CuSO_{4} $ are mixed with 20 kg of water. The mixture is then kept at $\displaystyle 20^{o}C $ until the maximum possible amount of $\displaystyle CuSO_{4} $ has dissolved. Any undissolved $\displaystyle CuSO_{4} $ is hydrated to form solid

$\displaystyle CuSO_{4}.5H_{2}O $ . Calculate the mass of undissolved solid.

Data: Molecular Masses:

$\displaystyle CuSO_{4}= 159.6 $

$\displaystyle CuSO_{4}.5H_{2}O = 249.7 $

Solubility at $\displaystyle 20^{o}C $: 20.7kg $\displaystyle CuSO_{4} $ per 100 kg water.

I am bit confused on how to do this, as I am no sure what I am suppose to be working out? Is it the amount of copper sulphate in the hydrated solid?

Any help appreciated.

Thank you.