Question

Such an oil drop falls 4.0 × 10−15

Answer 1

If an oil drop falls on water so that the thickness of the oil spot is $4\cdot10^{-15}$ m, and the area of the oil spot is 1 squared meter, the volume of the oil drop was

$$V=Ah=1\cdot4\cdot10^{-15}=4\cdot10^{-15}\text{ m}^3,$$

or about $4\cdot10^{-6}$ mm³.