Answer:
For now, I could tell you that it is not expected a numerical prediction based on experimental measurements to be exact. There are always experimental mistakes that add error to the numerical predictions, not only attributed to the one doing it, but also to the equipment used.
For instance, when a compound is weighed on a balance you should always notice it has uncertainty (e.g. +-1 kg), this means that if the balance says 40 kg, the weight could be between 39 and 41 kg.
Regarding the second question, when you add more values with the same experiment, you can obtain an average value and the most important thing, a deviation that would allow you to include your result in a range.
For the last question, I would need more context.
Have a nice day.
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