Answer:
51.65%
Step-by-step explanation:
Magnesium carbonate pentahydrate is

1 mole of the compound contains 5 moles of water. The water can be extracted by heating the the compound and allowing the water to condense.

1 mole of the compound contains 5 moles of water molecule
mole = mass/molar mass
1 mole of the compound weighs: 1 x 174.3903 = 174.3903 g
5 moles of water weighs: 5 x 18.01528 = 90.0764 g
Percentage by mass of hydrate in magnesium carbonate pentahydrate
= 90.0764/174.3903 x 100%
= 51.65%