Express all quantities in the same unit, then simplify by dividing by the common factor.<br><br>
&bull; 20&nbsp;min : 45&nbsp;min : 1&nbsp;hour<br>
&bull; 20&nbsp;min : 45&nbsp;min : 60&nbsp;min<br>
&bull; Divide each by 5: 4 : 9 : 12<br>
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Final Answer: 4 : 9 : 12