The formula for calcium carbonate is
CaCO3
So each mole of calcium carbonate contains one mole of calcium
Molar mass of calcium carbonate = 100g/mole
Atomic mass of calcium = 40g/ mole
Hence if 40g of calcium is present it means we have 100g of calcium carbonate
if 1 g of calcium is present it means we have 100/ 40 g of calcium carbonate
So if 1000 mg of calcium is present = 100 X 1000 / 40 mg of CaCO3 is present
= 2500 mg of CaCO3 in tum tablets