Muons produced by cosmic rays hitting air molecules originate at a mean altitude of15 km above sea level. They would never survive to sea level without time dilation becausetheir mean

lifetime in their rest frame is only 2 us. The mean muon energy measured at sealevel is about 4 GeV and the muon rest mass is about 0.1GEV/c². - What is the mean lifetime of these muons as seenbyan observer on the ground? At what fraction of c are these muons moving?