1) Let's start by writing this ratio, and then simplify whenever possible.
[tex]\frac{21}{30}=\frac{21\text{ :3}}{30\colon3}=\frac{7}{10}[/tex]2) There's no way to reduce it more than that, so we can stop it here.
3) Hence 21 dimes to 30 dimes is the same as saying you've got 7 dimes to 10 dimes since it expresses the same ratio.
