Question

An automobile gasoline tank holds 20 kg of gasoline. when the gasoline burns, 83 kg of oxygen is consumed and carbon dioxide and water are produced.

Answer 1

Missing question: What is the total combined mass of carbon dioxide and water that is produced? 
Chemical reaction: gasoline + O₂ → CO₂ + H₂O.
For this question use mass conservation law (mass of products is equal to mass of reagents): m(reactants) = m(products of reaction).
m(gasoline + oxygen) = m(carbon(IV) oxide + water).
m(carbon(IV) oxide + water) = 20 kg + 83 kg = 103 kg.

Answer 2

The total mass before the reaction equals the total mass after the reaction. Gasoline + O2 = CO2 + H2O. the law of the conservation of mass, the mass of the products must equal the mass of the reactants in a closed system. Therefore, assuming a closed system 20 kg + 82 kg = 102 kg = mass of reactants = mass of products.