The sum of the numerator and denominator of a fraction is 21, which is 0.4 after converting to a decimal. What is the original fraction?
一个分数的分子和分母的和是21，化成小数后是0.4，原来这个分数是多少．
<gadget id="calculator">2 + 5</gadget>
<output>7</output>

<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">21 * (2/7)</gadget>
<output>6</output>

<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">21 * (5/7)</gadget>
<output>15</output>

<gadget id="calculator">6 / 15</gadget>
<output>2/5 = around 0.4</output>

<result>2/5 = around 0.4</result>