Answer
$$1.67*10^{20} molecules$$
Work Step by Step
We know that one drop of water contains 1/20 g, which is .05 g. We know that the molar mass of water is 18.02 g/mol as well. This allows us to find:
$$ \frac{.05 g}{1}* \frac{1 mol}{18.02 g}* \frac{6.022*10^{23} molecules}{mol}\\=1.67*10^{20} molecules$$
