what is the percent composition by mass of water in a compound with a gram formula mass of 4905 g/mol

Answer 1

Do you hope to obtain a number out of this?
%H2O = (mass water/formula mass)*100 = ??
Without the mass of water or the formula of the compound, a number is not possible.

Answer 2

it only tells me that and that gypsum contains 2 moles of water for each 1 mole of calcium sulfate from CaSO4*2H20

Answer 3

That's all you need.
g H2O = 2 moles H2O = 2*molarmass H2O.
Then %H2O = (2*molar mass H2O/molar mass CaSO4.2H2O)*100 = ??

This looks simple enough; however, I don't believe CaSO4.2H2O has a molar mass even close to 4905 g/mol.