Explanation

(a)
Number of atoms in 24 g of $C = \frac{24}{12}\ \times 6.02 \times 10^{23}$
=$2 \times 6.02 \times 10^{23}$
Number of atoms in
56 g of $Fe = \frac{56}{56} \times 6.02 \times 10^{23}\$
Number of atoms in
26 g of $Al = \frac{26}{27}\ \times 6.02 \times 10^{23}$
$\approx 6.02 \times 10^{23}$
Number of atoms in 108 g of $Ag = \frac{108}{108}\ \times 6.02 \times 10^{23}$
$= 6.02 \times 10^{23}$