1. The problem statement, all variables and given/known data A muon is a subatomic particle with an average lifetime of 2.2 microseconds when stationary. In a burst of cosmic rays in the upper atmosphere, muons are observed to have a lifetime of 16 microseconds. What speed are they travelling at to achieve this? 2. Relevant equations Time Dilation, Length Dilation, Mass Dilation, along with a few astrophysics formulas 3. The attempt at a solution So I'm thinking this is relativity. I used the time dilation formula: tv=t0/Sqrt[1-(v2/c2)] and worked out the velocity to be 2.972 x 108 m/s My maths is fine (it doesn't need checking but check it if you want to). What I really need to know is if this is the correct process?