Explanation: One formula of Al2(SO3)4 refers to one "molecule" of the given salt. Now from the given formula, we can note that one formula of the salt has: 2 Al atoms 3 SO4 atoms
Each one of the 3 SO4 atoms has 4 oxygen atoms, therefore: number of oxygen atoms in one formula of Al2(SO4)3 = 4 * 3 = 12 atoms