Hi there!
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[tex]15cm[/tex] as a ratio of [tex]1 meter[/tex]
is : [tex]3:20[/tex]
⇒Did you know? : 1 meter can also be written as 100 cm.
If 1 meter can be written as 100 cm the ratio would be : [tex]\frac{15}{100} = \frac{3}{20}[/tex]
Now we can express it as 3:20.
Hope this helped you!