There are two paper tapes, one is 21 cm long and the other is 13 cm long. After cutting off a section of the same length from both paper tapes, the short paper tape is exactly the length of the long paper tape (8/13), then the cut What is the length of a paragraph.
有两张纸带，一条长21厘米，一条长13厘米，把两条纸带都剪去同样长的一段后，短纸带恰好是长纸带长度的(8/13)，那么剪去的一段长是多少．
<gadget id="calculator">8 / 13</gadget>
<output>8/13 = around 0.615385</output>

<gadget id="calculator">21 * (8/13)</gadget>
<output>168/13 = around 12.923077</output>

<gadget id="calculator">13 - (168/13)</gadget>
<output>1/13 = around 0.076923</output>

<gadget id="calculator">1 - (8/13)</gadget>
<output>5/13 = around 0.384615</output>

<gadget id="calculator">(1/13) / (5/13)</gadget>
<output>1/5 = around 0.2</output>

<result>1/5 = around 0.2</result>