1. The problem statement, all variables and given/known data The pion has an average lifetime of 26.0ns when at rest. For it to travel 10.0m, how fast must it move? 2. Relevant equations Lorentz velocity transformation? 3. The attempt at a solution I'm very lost... am I supposed to use u'x = (ux-v)/(1-vux/c2)? I thought I was following the lecture pretty well, but now that I'm trying to solve a problem, I have no idea what to do. After some more poking around, I used Δt=γΔt'. Now I have one equation and two unknowns (v and Δt). I know I am supposed to use a velocity equation to find v now, but I'm still not sure which eqn to use, and how.