During a combustion reaction, 12.2 grams of methane reacts with 14 g of oxygen. The reaction produces carbon dioxide and water. If 20 grams of water are produced, how many grams of carbon dioxide are produced?

The reaction produces 6 g of carbon dioxide.

The word equation is

methane + oxygen → carbon dioxide + water

12.2 g + 4 g → x g + 20 g

The Law of Conservation of Mass tells us the total mass of the reactants must equal the mass of the products. Then

12.2 g + 14 g = x g + 20 g

x = 12.2 + 14 - 20 = 6

The mass of carbon dioxide is 6 g.

Note: The answer can have no decimal places because you gave none for the masses of oxygen and water.