Well I had actually thought of using time dilation myself, I just couldn't figure out how to relate it to this problem. If I take the muons to be the home frame with the "earth traveling towards them" as Orodruin had pointed out that makes the 1.52us the proper time right? But there I don't know...
Question: Muons created in the upper atmosphere are sometimes able to reach the Earth's surface. Imagine that one such muon travels the 60km from the upper atmosphere to the ground (in the Earth's frame) in one muon half-life of 1.52us (in the muon's frame). How thick is that part of the Earth's...