If a muon is traveling at 70% of the speed of light, how long does it take to decay in the observer's rest frame (i. e., what is the observed lifetime τμ of the muon)? express your answer in microseconds to two significant figures.

The muon lifetime in the muon reference frame is


In the observer's rest frame, the muon lifetime is instead given by

where
is the relativistic factor, with v being the muon speed and c the speed of light.

Since the muon is traveling at 70 % of the speed of light, 

and the relativistic factor is


Therefore, the muon lifetime in the observer's rest frame is

Iagree c would be the answer