Simplify the ratio by dividing both numbers by their HCF.<br><br>
&bull; Lime water : Plain water = \(20 : 300\)<br>
&bull; HCF of 20 and 300 is 20.<br>
&bull; \(20 \div 20 = 1\), \(300 \div 20 = 15\)<br>
&bull; Ratio = \(1 : 15\)<br><br>
Final Answer: 1 : 15