How many gram atoms and number of atoms are present in $20g$ of calcium?

The correct option is B $0.5$ gram atoms and $3.01\times {10}^{23}$ atoms
Weight of calcium $=20$ g

Atomic weight of calcium $=40$ g/g atom

Number of gram atoms of calcium $=\frac{20g}{40g/gatom}=0.5$ gram atoms.

The Avogadro's number is $6.023\times {10}^{23}$ atoms/ g atom

The number of atoms of calcium $=0.5$ gram atoms $\times 6.023\times {10}^{23}$ atoms/ g atom
$=3.01\times {10}^{23}$ atoms