In a chemical reaction, the law of conservation of mass states that mass is neither created nor destroyed. Therefore, the total mass of the products must equal the total mass of the reactants.
In this case, the scientist starts with 30 grams of calcium carbonate (CaCO3). As it decomposes into carbon dioxide (CO2) and calcium oxide (CaO), the total mass of the products will also be 30 grams.
So, the correct response is:
30 grams