In a 10*g mass of an iron oxide there are 4*g of iron. How many grams of oxygen are there?

If I have £10-00 and I give you £4-00, then how many pounds do I have left?
You have formed $10 \cdot g$ of an oxide of iron; this could $F e O$, or $F {e}_{2} {O}_{3}$ or a mixed valence oxide. The point is that the constituent atoms are the metal and oxygen. If the iron content is $4 \cdot g$ then surely there must be a $6 \cdot g$ mass of oxygen.