sorry for this being so long I'm a rush but ask questions pls.
Explanation:
The question specifies ranges of possible lengths and masses. The length can vary from about 19.5 millimeters to almost 20.5 millimeters. The mass can vary from about 99.5g to almost 100.5g.
For more about my use of the terms about and almost, and the reason for the “.5” values, see the Notes at the end.
Now:
A smaller object with a given mass will have a higher density;
A larger object with a given mass will have a lower density;
An object of given size with a smaller mass will have a lower density;
An object of given size with a larger mass will have a higher density.
This could be summed up by saying that density varies inversely with size but varies directly with mass.
So the lowest density occurs with the largest length and the smallest mass, while the highest density occurs with the smallest length and the largest mass.
Within the bounds of error, the largest possible length is about 20.4999…mm while the smallest possible mass is 99.5g. So doing the density calculation with these numbers will give the lowest possible density.
Similarly, doing the density with a length of 19.5mm and mass of 100.4999…g will give the largest possible density within the error bounds.
Notes
The first issue is with the term “accurate to the given millimeter”. My interpretation follows the guidelines in Illustrative Mathematics (under Solution) and appears, from the text in that page, to be consistent with the Common Core (which is the way we teach math to kids these days in the U.S.)
In the context of measuring the diameter of a circle, it says:
Juan finds that the circle is 5 cm or 50 mm in diameter. Since the tape measure is accurate to the nearest millimeter, this means that the actual diameter is between 4.95 centimeters and 5.05 centimeters.
There’s a deeper issue associated with rounding. The most common rule is that 0.5 rounds up. Thus the quote above is not quite correct; the actual diameter is between 4.95 centimeters and just less than 5.05, but not 5.05 exactly which would round to 5.1 centimeters.
The example in the linked page probably doesn’t deal with this subtle issue because it’s for kids who have not yet been taught about things like repeating decimals or the conceptual complexity of perfect rounding. I dealt with it by specifying “…” in the values above.