Here is the question: If 1000 muons are incident from a height of 10 km above the earths surface, how much slower than the velocity of light must they be travelling in the rest frame of the Earths surface for 990 of them to be expected to arrive at the ground undecayed? The mean life of a muon is 2.2 microseconds. You can use equation: (gamma)(beta)=sqrt((gamma)^2+1) for (gamma) much greater than 1. I cant seem to work out how to do this. So far I used the decay equation to find the time taken for the 1000 muons to decay to 990 muons as 2.21x10^-8 seconds but i am not sure where to go after this, help will be greatly appreciated.