Question

The density of silver is 10.5 g/cm3 . what is its density in ng/(mm)3

Answer 1

First we convert each of the units separately (easier in calculation):
1 gm = 1000000000 n gm 
1 cm^3 = 1000 mm^3

Then we get the requirement:
10.5 g/cm3 x (1000000000 / 1000) = 10500000 ngm/mm3