After 20% of a rope is cut off, 4 meters remain, what is the total length of the rope?
一根绳子截去20%后，还剩4米，这根绳子全长多少米．
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

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

<result>5</result>