Question 836316
The speed is the same for the whole trip, so this stays constant.

Let d = distance from San Diego to Salt Lake City.


4 hours, traveled d-507 miles;
8 hours, traveled d-267 miles.


What is the distance traveled between the 8 hour completion and the 4 hour completion?
{{{(d-267)-(d-507)=-267+507=240}}} miles.


From 4 hours to 8 hours time completion is a 4 hour time quantity.
We can find the value for the constant speed now.
{{{(240/4)(miles/hour)}}}
{{{highlight(60(miles/hour))}}}
-
You wanted an expression but not necessarily a finished value.  You should be able to construct the expression for speed if that is what you wanted.


You can also find the value for d, because now you know some of the times, and you know the constant speed.


The first four hours:
{{{60*4=d-507}}}
{{{d=60*4+507}}}
{{{d=240+507}}}
{{{d=747}}}