a 30cm ruler
An ordinary ruler.
A metric ruler
A normal metric ruler!
A meter stick
A ruler is usually divided by: inches, centimeters, and millimeters.
The customary ruler uses both inches and centimeters.
You use a ruler or similar object that is marked with centimeters. If your ruler only has inches, measure the inches, then multiply with 2.54 to get centimeters.
It will be on the Centimeters
30 centimeters
It is centimeters and inches.
It is centimeters and inches.
It is centimeters and inches.