True or false: One gallon of gasoline weighs about 6 lb but produces about 20 lb of CO₂ when burned.

What the statement is really testing

It sounds impossible at first because it seems like you are getting more mass out than you put in. The key idea is that burning gasoline combines the fuel with oxygen from the air, so the products include mass that was not in the liquid gasoline.

Why the $CO_2$ mass is larger than the gasoline mass

In complete combustion, the carbon and hydrogen in gasoline react with oxygen: $$\text{hydrocarbon} + O_2 \rightarrow CO_2 + H_2O$$ Most of the mass of the $CO_2$ comes from oxygen atoms that were originally in $O_2$ from the air.

Quick number sense check

A typical emissions factor is about $8.9\ \text{kg }CO_2$ per gallon of gasoline, which is about $19.6\ \text{lb }CO_2$. Since a gallon of gasoline weighs about $6\ \text{lb}$, the statement “about 20 lb of $CO_2$” is a good approximation.

Final verdict

Because oxygen from the air is added during combustion, one gallon of gasoline can reasonably produce about $20\ \text{lb}$ of $CO_2$, so the statement is true.