You’ll need to be sure to count all the atoms in each side of the chemical equation.
The mass percentage of Na in NaCl is 39.0%
This means that if there's 100 mg of NaCl , 39 mg of the compound is Na.
For compound to contain 39 mg Na - 100 mg of NaCl should be present.
Therefore for the compound to contain 400 mg - 100/39 x 400 - 1025.6 mg
The mass of NaCl present should be 1.026 g
A.the bottom layers are compressed into ice