The denominator of (2/7) is increased by 21. To keep the size of the fraction constant, what is the numerator.
(2/7)的分母增加21，要使分数的大小不变，分子要多少．
<gadget id="calculator">7 + 21</gadget>
<output>28</output>

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

<result>4</result>