Halliday, Resnick & Walker — Relativity: Problem 53

53 In a high-energy collision between a cosmic-ray particle and a particle near the top of Earth’s atmosphere, 120 km above sea level, a pion is created. The pion has a total energy E of 4.00 x 10^5 MeV and is traveling vertically downward. In the pion’s rest frame, the pion decays 35.0 ns after its creation. At what altitude above sea level, as measured from Earth’s reference frame, does the decay occur? The rest energy of a pion is 139.6 MeV.