Subtract 24 from the denominator of (21/28), how much should be subtracted from the numerator to keep the fraction size constant.
(21/28)的分母减去24，要使分数大小不变，分子应减去多少．
<gadget id="calculator">28 - 24</gadget>
<output>4</output>

<gadget id="calculator">28 / 4</gadget>
<output>7</output>

<gadget id="calculator">21 / 7</gadget>
<output>3</output>

<gadget id="calculator">21 - 3</gadget>
<output>18</output>

<result>18</result>