The aluminum sulfate hydrate [Al2(SO4)3 (times) xH2O] contains 8.20 % Al by mass. Calculate x, that is , the number of water molecules associated with each Al2(SO4)3 unit.

RMM for Al2(SO4)3
=27*2+(32+4*16)*3
=342
RMM for H2O
=2+16
=18
Number of water molecules = n
Thus
27*2/(342+18n)=0.082
Solve for n to get 18 (to 2 signinficant digits).