Final answer:
Your friend may have used the incorrect unit scale, where an 8-inch line represents 800 feet, not 8 units. Additionally, units must be specified to avoid ambiguity. Using correct scale ratios and specifying units are critical for proper measurement.
Step-by-step explanation:
To explain to your friend why they are incorrect about the length of the line being 8 units, one could consider the unit scale being used. Since we know that to represent a line that is actually 500 feet long, we draw a scale of 5 inches, it becomes apparent that the scale is 100 feet per inch. Thus, an 8-inch line on this scale would represent an 800 feet line. Your friend might have missed that the scale factor should be applied to determine the actual length of the line being represented.
Another common mistake can occur if your friend forgets to specify units. For example, when stating 8 units, without specifying what those units are (feet, meters, inches, etc.), it leaves the statement ambiguous and incomplete. This is akin to when someone says they walk 12 units to school without indicating the actual unit of measurement.
By setting up a ratio, such as 0.5 inch representing 20 miles, we can determine the unknown length 'x' in miles if the scale length is 8 inches. If your friend did not use such a ratio or ignored units, they would be mistaken in their measurement.