The important relationship or constant we have to know here is the Avogadro's number. 6.02210^23 /mole. To answer this, we have to convert the mass of Calcium to moles by dividing it with its molar mass. The molar mass of Calcium is 40 g/mol.
\(moles Ca = 187 g * \frac{mol}{40 g} = 4.675 moles Calcium\)
Then we multiply the moles calcium with the Avogadro's constant.
4.675 moles Calcium 6.02210^23 /mole = 2.8210^24 atoms of calcium.
