This is the frirst SR problem in his book. I think the question is flawed. The solution assumes a speed of 3.00 E8 m/s to calculate Δt=333us since it traveled 100km. Next, 333us and 2.2us is used to solve for velocity using the time dilation formula. Velocity is reported to be .999978c. This does not make any sense. If you calculate Δt using C as the speed, then C should be the answer at the end, after you solve for velocity using the time dilation formula. Shouldnt .999978c be reported as 3.0 E8 m/s due to the significant figures? The answer you get at the end just ends up being whatever value you assumed for "very high speeds." Isn't this question impossible to answer unless you are given the actual velocity for the muon???? Thanks :-) PS. The next example in the book is the lengh contraction example for the muons reference frame. It uses 0.999978c as the velocity.