Ohh I see, alright thank you!
Are you sure you can simply use t=(d/u)? The electron is being accelerated across the potential difference, so its not constant velocity, which is why I feel you cannot use that eqn. The u that you have used the is maximum velocity, but it starts from 0 and accelerates to that velocity. It was not travelling at that velocity the whole time.
If the velocity were not constant your point would be absolutely right, but the velocity is constant for the journey between the gun and the screen, which is what my diagram covers. The question says that the screen is 0.4m away from the electron gun, which means that the screen is 0.4m away from the electron when it comes out of the gun, by which time its acceleration is complete.
This question is not helpfully worded. It should really say (1) the electron is accelerated by a voltage of 50V in an electron gun, and then (2) the electron travels a distance of 0.4m to a screen.
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