Hi, I have a question about this problem from the book; I am confused about how to start it.

Convert 2.1 mg to kg and then to metric ton and multiply with ratio  4g /metric ton. Dimensional analysis
2.1mg  x 1g/1000mg  x 1kg/1000g x 1 metric ton / 1000kg x 4g / 1  metric ton
        =  8.  * 10^-9 g = 8 ng

Ooh, I understand how to solve it now with dimensional analysis. Thank you