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.
