There are 0.857 moles in 24.0 grams of CO, carbon monoxide
This question is asking to connect two ideas together: the concept of the mole, the concept of molar mass (which is similar to gram formula mass).
When looking at the periodic table, every element has mass associated with it. This is called the gram formula mass or molar mass. Carbon is 12.0 grams/mole and oxygen is 16.0 grams/mole. The molar mass of carbon dioxide is 12.0 + 16.0 = 28.0 grams/mole
A mole (Avogadro's number) is
If you have 24.0 grams of CO, you do not one mole, but, rather less than that. So you divide your given value, 24.0 grams by 28.0 grams/mole to obtain the answer of 0.857 moles of CO