We can figure out its gamma factor from

Plugging in m = 9.1 x 10^^-31 kg, we find

The velocity of this electron is going to be VERY close to the speed of light. We might encounter round-off problems if we just plug these values into a calculator. Therefore, I'm going to use the BINOMIAL EXPANSION to derive an expression for the velocity of the electron which will not suffer from roundoff. My expression will be in terms of c, not in meters per second.

To find the the time it takes electron A to reach the Earth, I will use the BINOMIAL EXPANSION a second time:

Now, let's look at the other electron, which has twice as much energy.

We can figure out its gamma factor from

We can then derive the velocity of electron B

which is closer to c, as it should be.

The time it takes for electron B to reach the Earth is

Therefore, the high-energy electron B reaches the Earth slightly earlier than the low-energy electron A, by a difference of