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