What is 800 milliliters plus 300 millimeters?
You can not answer the question as written.
Millilitres is volume whilst millimetres is length.
However, if they were both millilitres you would have 1100 millilitres as your answer.
Here you have two numbers representing different quantities.
The first quantity represents a volume (a portion of space) while the second represents a length or distance. You cannot really add them together...it would be like to add two oranges plus three bananas...you get...5...bananges????