If 1 ml of water contains 20 drops, then the number of molecules in a drop of water is      

1 ml = 1g ( for H₂O)

18g $\to$6$\times$10²³ H₂O molecules

1g$\to$ ?

1 ml = 20 drops = $\frac{1}{3}$$\times$10²³molecules

1 drop = ?

$\frac{1}{20}\times \frac{1}{3}\times {10}^{20}=1.673\times {10}^{21}$molecules