If 1 mL of $H_{2}O$ contains 20 drops, then no. of molecules in a drop of water are

Volume of 20 drops = 1ml

⇒ Volume of 1 drop= $\frac{1}{20}\mathrm{ml}$ 

Mass of water = $\frac{1}{20}\mathrm{g}$ [since density= 1g/cm3]

∴ Moles of water =$\frac{1}{20}\times \frac{1}{18}$​= $\frac{0.05}{18}$

∴ Number of molecules = $\frac{0.05}{18}{\mathrm{N}}_{\mathrm{A}}=\frac{0.05}{18}\times 6.023\times {10}^{23}$= 1.673×10²¹ molecules