For this, you have to know the unit conversions. For metric, this is somewhat easier to remember. cm = centimeters. centi means 100th. One way to remember this one is that a cent is 1/100th of a dollar. So also is a centimeter 1/100th of a meter. A millimeter is 1/1000th of a meter. so a centimeter is 10 millimeters.
[tex]\frac{20}{1} \frac{cm}{1} * \frac{10}{1}\frac{millimeters}{cm} = 200 millimeters[/tex]
Therefore the solution is 200 millimeters.
