1. The problem statement, all variables and given/known data A beam of unknown particles travels at a speed of 2.5 x 10^8 m/s. When the particles are moving at this speed, their average lifetime is found to be 7,3 x 10^-8 s. What is their average lifetime when at rest? 2. Relevant equations Well, Lorentz transformation for time is an obvious one: t = t'*[itex]\gamma[/itex] 3. The attempt at a solution Well, I really have no idea where to pick this from. I know that when v = 0 => t = t'. I'm missing something here... Thanks in advange.