If the momentum of electron is changed by P, then the de Broglie wavelength associated with it changes by 0.5%. The initial momentum of electron will be :

The de-Broglie‟s wavelength associated with the moving electron  $\lambda =\frac{h}{p}$, where p = momentum of the electron.

Differentiating, $\frac{d\lambda }{\lambda }=-\frac{dp}{p}$

Assuming small changes,

$\frac{\left|∆\lambda \right|}{\lambda }=\frac{\left|∆p\right|}{p} \Rightarrow p=\frac{\left|∆p\right|}{\left({\displaystyle \frac{\left|∆\lambda \right|}{\lambda }}\right)}$

Using values given in problem, 

$\text{Initial momentum, }{p}_i=\frac{P}{{\displaystyle \frac{0.5}{100}}}=200P$