If you were asked to convert 25 mg to the unit hg, which of the following would be the first fraction used in the conversion

You can construct your own conversion factors from the definition of units. 1 g = 1000 mg,, and 1 hg = 100 g. From the first, you have the fraction factor 1 = 1 g / 1000 mg, from the second you have the fraction factor 1 = 1 hg / 100 g. So, you can goe from 25 mg to hg by doing 25 mg * 1 g / 1000 mg * 1 hg / 100 g. The fraction 1 g / 1000 mg converts the 25 mg into grams, and the fraction 1 hg / 100 g converts grams into hg. So, the answer to the question is 1 g / 1000 mg.

Abbreviated as mg-25.