Is this a misprint in my SR homework?

[sigma_]

Homework Statement

"In frame of reference S, an electron moving along the x-axis has
energy 3mc² and momentum magnitude √(8)mc
Use the transformations of energy and momentum to find the energy and momentum magnitude observed in
frame S′ moving with speed 4c/5 relative to S in the positive x-direction."

Homework Equations

How can the electron have a proper velocity of c?

If relativistic velocity is dx/dτ = γdx/dt, and relativistic momentum is simply the mass time the relativistic velocity, how is m(γ(dx/dt)) = √8mc?

The Attempt at a Solution

I solved the problem correctly, I am just unsure of a detail in the wording of the problem.

 

[TSny]

Hello

How can the electron have a proper velocity of c?

How do you get that the electron has a proper velocity of c?

If relativistic velocity is dx/dτ = γdx/dt, and relativistic momentum is simply the mass time the relativistic velocity, how is m(γ(dx/dt)) = √8mc?

What is it about m(γ(dx/dt)) = √8mc that you find disconcerting?