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= Number of total story points / One iteration
Velocity is a measurement of how much the team gets done in an iteration ( called Sprint in Scrum ). Velocity is what actually got done in the last iteration not what is planned.
In Scrum, it is measure in Story points. Each feature in scrum is a story. A story has points. Points can be anything you come up with.
Examples are 1, 2, 4, 8, 16
5, 10 15 and so on.
A story depending on its complexity is given certain story points. So if the team does 6 stories that are 8 story points that iteration, the team’s velocity is 48 story points.
Agile Practitioners Guide 
You have learned in school that speed is something related to something else, you cannot say that velocity is 64 points.
The correct is 64 points per iteration, yes, you have too include that last part, you cannot imagine how much confusion this creates, the example is good, nothing to comment upon it.
Handling of stories not completed is important since these tend to skew the numbers when reporting. It is easy for a team that is struggling to count carryovers as part of the velocity for the next iteration. When they do, each iteration seems to have a steady velocity, when the reality is that they do not.
Carryovers should be measured and managed appropriately. They are a warning sign that the team is in trouble. Velocity counts should be lowered, or reported separately in order to paint the right picture.
poor example
show calculations
give full example