Answer
Molecular mass of Na$_{3}$AlF$_{6}$ =
3x atomic mass of Na + atomic mass of Al + 6 x atomic mass of Fluorine
= (3 x 22.9898) + (26.9815) + (6 x 18.998) = 209.9413%
Mass %age of Na = 68.9694/209.9413 x 100 = 32.8517%
Mass %age of Al = 26.9815/209.9413 x 100 = 12.8519%
Mass %age of Fluorine = 113.988/209.9413 x 100 = 54.2963%
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