Question 709712
{{{drawing(250,300,-1,9,-1,11,
line(0,0,0,11),line(0,0,9,0),
arrow(0,8,0,11),arrow(6,0,9,0),
locate(-1,10.8,miles),
locate(7.7,1,hours),
blue(line(0,0,5,5)),
blue(line(6,5,5,5)),
blue(line(6,5,8,10)),
blue(line(-0.3,5,0.3,5)),locate(-1,5,7.5),
blue(line(-0.3,10,0.3,10)),locate(-1,10,15),
blue(line(5,-0.3,5,0.3)),locate(4.1,-0.2,2.5),
blue(line(6,-0.3,6,0.3)),locate(6,-0.2,3),
blue(line(8,-0.3,8,0.3)),locate(8,-0.2,4)
)}}} Your walk up the 7.5-mile trail, at an even 3 mile per hour pace,
is represented by the first up-sloping blue segment.
At each point in that line the distance in miles is 3 times the time in hours.
It is a straight line, because your speed was constant 
After 2.5 hours, the total distance covered was 7.5 miles.
At that point you stopped to rest and eat lunch for half an hour.
During that rest/lunch period (2.5 to 3 hours into your trip),
the total distance covered was a constant 7.5 miles,
and your speed was a constant zero (because you were not walking).
The lunch/rest part is represented by the short horizontal segment.
After lunch, for the next hour, you ran downhill down the same trail.
Because, it was downhill,
it was not a great effort to keep a steady 7.5 mile per hour pace
all the way (7.5 miles) to your starting point.
When you finished you had covered the 15 mile distance in just 4 hours.
The steep upper blue segment represents your downhill run.
It is a straight line because your speed was constant,
and it is steep, because you were going fast.