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 $\times$ 10⁵ ms^$-$1. The speed with which electron enters the p side will be ${x \over 3} \times {10^5}$ ms^$-$1 the value of x is _____________.

(Given mass of electron = 9 $\times$ 10^$-$31 kg, charge on electron = 1.6 $\times$ 10^$-$19 C.)