If you want a simple average, you can add all the speeds you see and divide them by the number of how many they are.
So firstly you have to calculate the speed of the first point, which is:
20 miles/0.5 hour
That means: 40 miles/hour
That would be your first "speed"
Then you calculate all of them :speed1+speed2+speed3 = something.
As an example: 20miles/hr + 40miles/hr+0miles/hr = 60miles/hr
Then, you divide it by how many "speeds" they are: 60miles/hr divided by 3 if I continue my previous example.
It is my first answer, and I hope I could help you a bit!
Sorry for the awkward English, as it is not my mother tongue.
