Science
posted by Elizabeth .
Metric System
How do I know when to multiply and when to divide when converting to metric or from?
Sample 450km=______mi

OK, good question. You know whatever units you start with will have to be divided by something across the fraction bar.
450km ( xxx/yyy) so km will be the units of yyy.
450km* 1000m/1km will work. Now I am in meters, and I need to get to miles. Hmmm. I know there is 1609m per mi, so
450km*1000m/1km * here I put m in the denominator so it divides out...
450km*1000m/km * 1mi/1609m and that does it. 
Thank you, I'm going to practice and see how I do.