What is the percent by mass of water in the hydrate? %

Sorry I can't tell you the answer but, I can tell you something ...

Dividing the mass of the water lost by the mass of hydrate used is equal to the fraction of water in the compound. Multiplying this fraction by 100 gives the percent water in the hydrate.

How to find it?

Divide the mass of the water lost by the mass of hydrate and multiply by 100. The theoretical actual percent hydration percent water can be calculated from the formula of the hydrate by dividing the mass of water in one mole of the hydrate by the molar mass of the hydrate and multiplying by 100.