Probably the question is about mass: what percent does aluminium constitute of the mass of a molecule `Al_2 (S O_4)_3 ?`
To calculate this, we should know the atomic masses of `Al` (aluminium), `S` (sulfur) and `O` (oxygen) atoms in some units. We can choose any units but the same for all types of atoms.
The most convenient unit in this situation is the relative atomic mass. It is defined as `1/12` of the mass of an atom of carbon-12. We may look the periodic table to see that
`m(Al) approx 27,` `m(S) approx 32,` `m(O) approx 16.`
Then the mass of a molecule of `Al_2 (S O_4)_3` is
`2*m(Al) + 3*(m(S)+4*m(O)) approx 342.`
Aluminium in such a molecule has the mass `2*m(Al) approx 54.` Thus the percent in question is `54/342 *100% approx15,79%,` so the answer is 15.8%.