Hi grandh01,

Always try to prepare equations when you are
given problems with some non numerical values.

In this question you are told that the time to cover
a distance covered at 55 mph is 1 hour less than the time
to cover the same distance in 50mph.

I>
So you need to consider the time as 't' because
speed*time=distance, (We know speed but not time, and distance being equal we equate)
Now,

50*t=55(t-1) (Since at 55mph he would have covered the same distance in 1 less than t hours)
If we equate we get the value.
Hence it is sufficient.

II>
Half distance in 11 hours at 25 mph.
So distance is
25*11=275miles
hence if half is known we can calculate full.

Answer : D

Small tip : Never sit down and calculate if you know the answer is achievable. Its simply
a waste of time.