Compare:
- The front of the train has a speed of 23 m/s
The front of the train has a speed of 23 m/s
The front of the train has the speed of 23 m/s
- The front of the train has the speed of 23 m/s
If you ask #2, the reader will understand that there are a very narrow set of possible values. It would make the reader think this is a problem with, for example, a binary choice, where we're comparing Train A with speed 23 m/s and Train B with speed 27 m/s. Reason: "The" is used to refer to something previously mentioned.
Note that the following is acceptable and won't be misunderstood:
- The front of the train has speed 23 m/s.
In the discussion, you asked
Why would we say, "Forty-five percent of the population has a blood type of O?" After all, if anything can be said to be fixed, it is a person's blood type.
Same thing. If you use "the," the reader will take that as an indication that you are analyzing a situation where only two (or three) blood types are possible. (That assumption would have been established in the set-up of the problem, in earlier sentences in the article or homework or exam problem.)
If you want to discuss "He has an emotion of approval" I suggest writing a separate question.
