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 $\underline{\hspace{2cm}}$. (Given mass of electron = 9 $\times$ 10 $-$ 31 kg, charge on electron = 1.6 $\times$ 10 $-$ 19 C.)