Explanation

(a)
Number of atoms in 40 kg$= \frac{40 \times 10^{3}g}{6.644 \times 10^{- 23}\ g}$
($\because$ Weight of an atom=$6.644 \times 10^{- 23}$g)
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 6.02 \times 10^{26}$
$\therefore$ Number of gram atoms of element in 40 kg
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = \frac{6.02 \times 10^{26}}{6.02 \times 10^{23}} = 10^{3}$