How much is added to the numerator and denominator of (1/13), the original fraction becomes (2/5).
(1/13)的分子分母都加上多少，原分数就变成(2/5)．
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(2/5) * 13</gadget>
<output>26/5 = around 5.2</output>

<gadget id="calculator">(26/5) - 1</gadget>
<output>21/5 = around 4.2</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">(21/5) / (3/5)</gadget>
<output>7</output>

<result>7</result>