A particle of mass M moves through the lab at 0.6c. Suddenly it explodes into two fragments. First

Question

fragment, with mass 2M/3
, moves at 0.8c in the same direction that the original particle had been moving.
Determine the velocity (magnitude and direction) of the second fragment. What is the nature (or name) of
the second particle?

Answer 1

Are your quoted masses (M; 2M/3) rest masses or masses in the laboratory frame of reference?

Momentum must be conserved in the laboratory rest frame as well as that of the incoming particle. Mass does not have to be conserved. (There could have been a conversion of mass to kinetic energy)

I do not see how the particle can be identified from the limited information provided.