How many mg of calcium are present in a 500-mg tablet of calcium carbonate? ?

The molar mass of calcium carbonate is 100 mg/mmol, so the number of moles is:

moles CaCO3 = 500 mg / (100 mg/mmol) = 5 mmol

We see that there is 1 mole of Ca per 1 mole of CaCO3, therefore:

mass Ca = 5 mmol * (40.08 mg/mmol) = 200.22 mg