When carbon is burned in air, it reacts with oxygen to form carbon dioxide. When 25.2 of carbon were burned in the presence of 80.6 of oxygen, 13.4 of oxygen remained unreacted. What mass of carbon dioxide was produced?

Answer 1

No units again on any of the numbers.
Subtract oxygen initially - oxygen at the end. That turns it into a stoichiometry problem. Use the link from the last post.