[a.n]

One answer would be to heat at low flame. Any other answers?
I thought this would be right but was marked wrong:
"Heat and reweigh the hydrated salt at shorter more regular intervals just untilconstant mass is achieved. " why is that wrong?

[Borek]

That's not a way of avoiding, that's a way of detecting decomposition when it is already too late.

[Nitishajack]

heat it under vacuum would also be correct.

Your answer was wrong because if the heat at which you dehydrate is also the heat at which you decompose you will not achieve a constant weight if the decomposition products are volatile. Your answer was paritial right if I understand your rationale was to minimize the amount of time you spent heating it...that is a good proposal.