Question

3Fe+4H2O-->Fe3O4+4H2 What is the mole ratioof Fe3O4 to Fe?

Answer 1

For every 3 Fe there is only 1 Fe3O4 producing.
So, ratio would be: Fe3O4 / Fe = 1 / 3

In short, Your Answer would be: 1 : 3

Hope this helps!