How many milliliters equals 40 milligrams?

Answer 1

This question cannot be answered sensibly. A millilitre is a measure of volume, with dimensions [L3]. A milligram is a measure of mass, with dimensions [M]. Basic dimensional analysis teaches that you cannot convert between measures with different dimensions such as these without additional information.

As a simple mental exercise consider 40 mg of air and 40 mg of water. They will have very different volumes.