“Five or more, round up” is a picture before it is a rule. Here is the picture.
Number
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Mean, median and mode from one sorted list — plus the range, which measures spread rather than average.
The same digit is worth a different amount depending on where it sits.
Built one letter at a time, with the subtractive pairs like IV and CM explained as they appear.
Round first, work it out in your head, then check the real answer is close.