((x)/(0.5))=((x-18)/(0.3)).
((x)/(0.5))=((x-18)/(0.3))．
<gadget id="calculator">18 * 0.5</gadget>
<output>9</output>

<gadget id="calculator">0.5 - 0.3</gadget>
<output>0.2</output>

<gadget id="calculator">9 / 0.2</gadget>
<output>45</output>

<result>45</result>