Atoms are the smallest particles of an element that can take part in a chemical reaction. The compound 3(NH4)SO4 has. 3 nitrogen atoms, 12 hydrogen atoms, 1 sulfur atom and 4 oxygen atoms. To get atoms of nitrogen we multiply 1 by 3 outside the brackets to get 3, for hydrogen atoms, inside the brackets we have 4 atoms then we multiply by 3 to get 12. Sulfur atom is only one since there is no brackets or subscript encompassing it, for oxygen, there are 4 atoms as a result of multiplying one by the subscript 4.
Therefore, she should not have multiplied the sulfur atoms by the subscript 4