# How do you balance FeO_3(s) + CO(g) -> Fe(l) + CO_2(g)?

$F e {O}_{3} + 3 C O \to F e + 3 C {O}_{2}$
The transition from carbon monoxide to carbon dioxide uses up one part of oxygen. So to use the three parts of oxygen in $F e {O}_{3}$ combine with $3$ parts carbon monoxide.