Subtract 3 from the numerator of (15/20), how much should the denominator be reduced to keep the size of the fraction constant.
把(15/20)的分子减去3，要使分数的大小不变分母应减少多少．
<gadget id="calculator">15 - 3</gadget>
<output>12</output>

<gadget id="calculator">12 / 15</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">20 * (4/5)</gadget>
<output>16</output>

<gadget id="calculator">20 - 16</gadget>
<output>4</output>

<result>4</result>