Answer:
80 cm
Step-by-step explanation:
You can only add measurements that have the same units, so you must convert mm to cm or vice versa to compute the sum of the measures.
500 mm is 50 cm, so the two lines together total ...
30 cm + 50 cm = 80 cm
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Comment on significant digits
There is always some ambiguity as to the significance of trailing zeros to the left of the decimal point. That ambiguity can be resolved by putting a decimal point with the number, indicating that the 1s place digit is significant.
These measurements can be written in terms of meters (m), decimeters (dm), centimeters (cm), millimeters (mm), or even larger and smaller units. That is, it is possible to unambiguously express the precision of the measurements by appropriate choice of units.
If Jesse has done that, then one measurement has a precision of 1 cm, and the other a precision of 1 mm. The sum of them should be expressed with a precision of 1 cm if significant digits are of importance. (Your teacher may argue based on the lack of the decimal point described above.)
