Answer
$n=1.3 \times 10^{17} $
Work Step by Step
Setting up a simple proportionality (with the areas), the number of atoms is estimated to be
$$
n=\frac{\pi\left(2.5 \times 10^{-2} \mathrm{~m}\right)^2}{0.015 \times 10^{-18} \mathrm{~m}^2}\\=1.3 \times 10^{17} .
$$
