Jennifer, since Taryn was two times faster, that means she got there in half the time, right? So, the simplest thing to do is let t = Taryn's time. If that's the case, then Marcus' time (m) = 2t.
We can write that m = 2t
Now, Taryn's time was also 4 hours less than Marcus', so m = t+4.
Since m = m, then 2t = t +4, so t = 4 hrs.
That means that m = 8 hrs.
Those are our times. If it took Marcus 8 hours to drive 208 miles, and we let s = the speed, then our equation is 8s = 208, so s = 26 mph.
Since Taryn's time was twice as fast, her speed was 52 mph.
I hope that helps!