Cosmic ray, relative velocity

[najisalem2009]

Homework Statement

A cosmic ray travels 60km through the Earth's atmosphere in 400μs, as mesured by experimenters on the ground.
a. Calculate the relative velocity between the cosmic ray's intertial reference grame and the Earth's inertial reference frame.

b. How long does the journey take according to the cosmic ray?

c. According to the cosmic ray, how thick is the Earth's atmosphere?

Homework Equations

Time dilation equation
Δt=Δt'/$\sqrt{}1-v²/c2$

Length contraction
L=L'[itex]\sqrt{}1-v²-c²

The Attempt at a Solution

Answers a) v=0.5c, b) t_ray=346μs, c) L_ray=52km

Im very confused on how to approach this question, can anyone point me in the right direction.

 

[cepheid]

Well, part (a) is just asking you to figure out how fast the cosmic ray is moving as measured by the people on Earth. How do you figure out speed given distance and time?