Hello everyonea. I was studying special relativity and i got stuck on an example. Here it is:'to an observer,two bodies of equal rest mass collide head on with equal but opposite velocities 0.8c and cohere. To a second observer,one body is initially at rest. Find the apparent velocity of the other moving mass before the collision and compare its initial energy in the 2 frames of reference.' i am trying to find their relative velocity but am finding a value greater than c. Can somebody help me on this or give some tips on how i should start. Thanks for understanding.