A drop of oleic acid solution holds 1.5 × 10⁻⁵ cm³ of pure acid…

Step 1. Thickness. $\dfrac{1.5 \times 10^{-5}\ \text{cm}^3}{60\ \text{cm}^2} = 2.5 \times 10^{-7}\ \text{cm}$.

Step 2. In metres. $2.5 \times 10^{-7}\ \text{cm} \div 100 = 2.5 \times 10^{-9}\ \text{m}$.