In 4:9, if the former term of the ratio is reduced by 2, how much should the latter term be reduced to keep the ratio constant.
在4：9中，如果比的前项减少2，要使比值不变，比的后项应该减少多少．
<gadget id="calculator">9 / 2</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">9 - (9/2)</gadget>
<output>9/2 = around 4.5</output>

<result>9/2 = around 4.5</result>