What is the formula of magnesium phosphate?

Think it through first, then pick an option.

Answer and explanation

Correct answer: d Mg₃(PO₄)₂

The book's example: Mg is 2 and PO₄ is 3; crossing over gives Mg₃(PO₄)₂.

Why the other options are wrong

a Mg(PO₄)₂: Phosphate's 3 did not go under Mg; the right formula is Mg₃(PO₄)₂.
b Mg₂(PO₄)₃: Each valency sits under its own symbol; they were not crossed over.
c Mg₂(PO₄): The numbers do not fit: Mg is 2 and PO₄ is 3, so three Mg and two PO₄.