How fast must a pion be moving in order to travel 15 m before it decays? The avg lifetime, at rest, is 2.6 x 10^-8 s. The answer is supposed to be 0.89c This sounds so easy. Like, just plugging in the formula but I'm not getting it. I haven't done relativity in so long. I think I must've forgotten more than I thought. I can't even find my notes now. Can someone please tell me how to get to the answer?