A sample of sandstone consists of silica, SiO2, and calcite, CaCO3. When the sandstone is heated, calcium

Question:

A sample of sandstone consists of silica, SiO2, and calcite, CaCO3. When the sandstone is heated, calcium carbonate, CaCO3, decomposes into calcium oxide, CaO, and carbon dioxide.
CaCO3(s) → CaO(s) + CO2(g)
What is the percentage of silica in the sandstone if 18.7 mg of the rock yields 3.95 mg of carbon dioxide?