10t car moving at 0.8m/s strikes a car and go together at a velocity 0.5m/s till it comes to rest?

Physics is a very precise subject. "10t car" is not a standard term. One would assume the 't' means 'tons' but that is not a metric unit. 'Before' is not spelled 'b4'. You just can't be sloppy in physics: not even a little bit.

Oh, and you have not given enough information to calculate an answer.