The muon is an unstable particle that spontaneously decays into an electron and two neutrinos. If the number of muons at t 0 is N0, the number at time t is given by N N0et/, where is the mean lifetime, equal to 2.2 s. Suppose the muons move at a speed of 0.95c and there are 5.0 104 muons at t 0. (a) What is the observed lifetime of the muons? (b) How many muons remain after traveling a distance of 3.0 km?