\(10\, \text{cm}=1\cdot 10^{-4} \, \text{km} = 0.0001 \, \text{km}\)$$t=\frac{s}{v}=\frac{10^{-4}\, \text{km}}{3\cdot 10^{5}\frac{\text{km}}{\text{s}}}=\frac{1}{3}\cdot \frac{10^{-4}}{10^{5}} \, \text{s}=\frac{10}{3}\cdot 10^{-10} \, \text{s}$$ Also 1/3 Nanosekunde.