5: 9 is added to the latter term of 9, to keep its ratio constant, what should be the former term.
5：9的后项加上9，要使它的比值不变，前项应该多少．
<gadget id="calculator">9 + 9</gadget>
<output>18</output>

<gadget id="calculator">18 / 9</gadget>
<output>2</output>

<result>2</result>