12. What volume of carbon dioxide contains the same number of molecules as 20.0 mL of oxygen at the same conditions?

Y ou need to use Avogadro's Law, normally taught in chemistry.

Avogadro's law (sometimes referred to as Avogadro's hypothesis or Avogadro's principle) is a gas law named after Amedeo Avogadro who, in 1811 hypothesized that "Equal volumes of ideal or perfect gases, at the same temperature and pressure, contain the same number of particles, or molecules." Thus, the number of molecules in a specific volume of gas is independent of the size or mass of the gas molecules.

So the answer to your query is 20.0ml of carbon dioxide [ or ANY other gas] at the same conditions