Add 21 to the former term of 7:4, to keep the ratio unchanged, how much should be added to the latter term.
给7：4的前项加上21，要使比值不变，后项应加上多少．
<gadget id="calculator">4 * 4</gadget>
<output>16</output>

<gadget id="calculator">16 - 4</gadget>
<output>12</output>

<result>12</result>