We have found that for our compund that is 70.0% Fe and 30.0% O the ratio of moles is:

1.25 moles Fe

1.88 moles O

Since chemical formulas use numbers like 1, 2, 3 and not numbers such as 1.25 or 1.88, we need to convert the ratio 1.25 : 1.88 to a whole number ratio. Let's do that next.