In a high-energy collision among a cosmic-ray particle and

Question

In a high-energy collision among a cosmic-ray particle and a particle near the top of Earth's atmosphere, 128 km above sea level, a pion is created. The pion has a total energy E of 1.81 x10⁵ 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 gauged from Earth's reference frame, does the decay occur? The rest energy of a pion is 139.6 MeV.