001mg=.001g=.000001kg Notice I placed the zeros on front of the milligrams so you can see the relation.
1 pill =889mg so 100 pills=88900mg or 88.9grams
Now move the decimal place 6 times to the left like on the .000001 kg.
88900 mg = 0.0889 Kg you can omit the zeros after the 9