Multiply the latter term of 4:9 by the smallest composite number, how much should be added to the former term to keep the ratio constant.
把4：9的后项乘最小的合数，要使比值不变，前项应加上多少．
<gadget id="calculator">9 * 4</gadget>
<output>36</output>

<gadget id="calculator">36 - 9</gadget>
<output>27</output>

<result>27</result>