4: If the former term of 9 is increased by 8, how much should the latter term be increased to keep the ratio constant.
4：9的前项增加8，要使比值不变，后项应增加多少．
<gadget id="calculator">4 + 8</gadget>
<output>12</output>

<gadget id="calculator">12 / 4</gadget>
<output>3</output>

<gadget id="calculator">9 * 3</gadget>
<output>27</output>

<gadget id="calculator">27 - 9</gadget>
<output>18</output>

<result>18</result>