If 1.8×10^-4cm3 of oil spreads to form a patch of area 150cm2, calculate the number of molecules in the drop.
no idea. All we know is that the thickness is
volume/area = 1.8*10^-4/150 = 1.2 * 10^-6 cm
Unless you know how many molecules high that is, you have no way to know how many molecules are there.