Question

An electron with (rest mass mo) moves with a speed of 0.8 c. Its mass when it moves with this speed is

Answer 1

The mass of an object moving at any speed is the same as the mass of that object moving at any other speed, including at rest relative to some inertial frame of reference.

In the bad old days, there was something called relativistic mass, mr, which was related to something called rest mass, m0, by a Lorentz factor:

    mr=mo1−v2/c2−−−−−−−−√

where v is the speed relative to the frame in which the rest mass is measured.

Nowadays we simple refer to the inertial mass. Inertial mass is frame-independent and does not vary with speed.

The bad old days answer to your question is that the Lorentz factor for v=0.8c is

    11−0.82−−−−−−−√=10.36−−−−√=53

a notional increase of two-thirds or about 67%.