Expand the latter term of 7:9 to 3 times of the original, how much should the former term be increased to keep the ratio constant.
把7：9的后项扩大到原来的3倍，要使比值不变，前项应增加多少．
<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<gadget id="calculator">21 - 7</gadget>
<output>14</output>

<result>14</result>