4: Add 12 to the latter term of 3, how much should be added to the former term to keep the ratio unchanged.
4：3的后项加上12，要使比值不变，前项应加上多少．
<gadget id="calculator">3 + 12</gadget>
<output>15</output>

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

<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<result>16</result>