Add 10 to the denominator of (2/5), how much should be added to the numerator to keep the size of the fraction unchanged.
(2/5)的分母加上10，要使分数大小不变，分子应加上多少．
<gadget id="calculator">5 + 10</gadget>
<output>15</output>

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

<gadget id="calculator">2 * 3</gadget>
<output>6</output>

<gadget id="calculator">6 - 2</gadget>
<output>4</output>

<result>4</result>