henry has 68 miles to destination after 45 minutes and 51.5 miles to destination after 71 minutes of driving. How many miles to his destination after 79 minutes of driving

Let d represent the distance of the destination from the starting point.

After 45 min, Henry has already driven d-68 miles.  After 71 min., he has already driven d-51.5 miles.

So we have 2 points on a straight line:

(45,d-68) and (71,d-51.5).  Let's find the slope of the line thru these 2 points:

                                 d-51.5 - (d-68)         16.5 miles
slope of line = m = ----------------------- = ------------------
                                     71 - 45                   26 min

Thus, the slope, m, is   m = 0.635 miles/min

The distance to his destination would be d - (0.635 miles/min)(79 min), or 

d - 50.135 miles.  We don't know how far his destination is from his starting point, so represent that by "d."

After 45 minutes:  Henry has d - 68 miles to go;

After 71 minutes, he has        d - 51.5 miles to go; and

After 79 minutes, he has         d -  x miles to go.  We need to find x.

Actually, much of this is unnecessary.  Assuming that Henry's speed is 0.635 miles/ min, and knowing that there are 8 minutes between 71 and 79 minutes, we can figure that the distance traveled during those 8 minutes is

(0.635 miles/min)(8 min) = 5.08 miles.  Subtracting thix from 51.5 miles, we conclude that after 79 minutes, Henry has (51.5-5.08), or 46.42, miles left before he reaches his destination.