What is 20 grams into 200 millimeters?

A gram is a measure of mass and millimeter is a measure of length--they are different units of measurement so they can not be converted between one another or cancel each other out.

If you would like to divide 200 millimeters by 20 grams, your answer would be 10 mm/g or 0.01 m/g. Not that you must keep the units in order to accurately represent the quantities being divided.