Enlarge the latter term of 5:9 to 3 times of the original, how much should the former term be increased to keep the ratio constant.
把5：9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">5 * 3</gadget>
<output>15</output>

<gadget id="calculator">15 - 5</gadget>
<output>10</output>

<result>10</result>