A potential barrier of $0.4 \mathrm{V}$ exists across a p-n junction. An electron enters the junction from the $n$-side with a speed of $6.0\times {10}^5{ \mathrm{m} \mathrm{s}}^{-1}$. The speed with which electron enters the $p$ side will be $\frac{x}{3}\times {10}^5{ \mathrm{m} \mathrm{s}}^{-1}$, then the value of $x$ is _____.: (Given mass of electron $=9\times {10}^{-31} \mathrm{kg}$, charge on electron $=1.6\times {10}^{-19} \mathrm{C}$.)

Hence, the correct answer is 14.