If 1 ml of water contains 20 drops. Then no. of molecules in a drop of water is 

$22400\mathrm{ml}$ of water contains $=6.023\times {10}^{23}$molecules 

lml of water contains $=\frac{6.023\times {10}^{23}}{22400}$ molecules

= 20 drops

:. 1 drop of water will contain $=\frac{6.023\times {10}^{23}}{224\times 2\times {10}^3}$

$=1.344\times {10}^{18}$ molecules