Divide the numerator of (9/21) by 3, how much should be subtracted from the denominator to keep the fraction constant.
把(9/21)的分子除以3，要使分数大小不变，分母应减去多少．
<gadget id="calculator">9 / 3</gadget>
<output>3</output>

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

<gadget id="calculator">21 - 7</gadget>
<output>14</output>

<result>14</result>