Add 6 to the former item of 2:3, to keep its ratio unchanged, how much should be added backward.
把2：3的前项加上6，要使它的比值不变，后向应加上多少．
<gadget id="calculator">4 * 3</gadget>
<output>12</output>

<gadget id="calculator">12 - 3</gadget>
<output>9</output>

<result>9</result>