# How many grams of calcium carbonate, CaCO_3, contain 48 grams of oxygen atoms?

A $100.09 \cdot g$ mass of calcium carbonate contains $48 \cdot g$ of oxygen atoms.
In one mole of calcium carbonate, there are $40.08 \cdot g$ calcium metal, $12.01 \cdot g$ of carbon, and $3 \times 15.999 \cdot g = 48 \cdot g$ of oxygen. From where did I get these masses? Did I know them off the top of my head?