Answer
According to the muon, the thickness of the atmosphere is $~1.47~km$
Work Step by Step
Let $L_0 = 60~km$
We can find the thickness $L$ according to the muon reference frame:
$L = L_0~\sqrt{1-\frac{v^2}{c^2}}$
$L = (60~km)~\sqrt{1-\frac{(0.9997~c)^2}{c^2}}$
$L = (60~km)~\sqrt{1-(0.9997)^2}$
$L =1.47~km$
According to the muon, the thickness of the atmosphere is $~1.47~km$
