In 25:15, if the preceding item is subtracted by 20, the ratio should remain unchanged. How much should be subtracted from the latter item.
在25：15中，如果前项减去20，要使比值不变．后项应减去多少．
<gadget id="calculator">25 - 20</gadget>
<output>5</output>

<gadget id="calculator">25 / 5</gadget>
<output>5</output>

<gadget id="calculator">15 / 5</gadget>
<output>3</output>

<gadget id="calculator">15 - 3</gadget>
<output>12</output>

<result>12</result>