A rope was used up 20% and then connected to 3.5 meters, the result was 4 meters shorter than the original. How long is the rope in meters?
一根绳子用去20%后接上3.5米，结果比原来还短4米．绳子原来长多少米．
<gadget id="calculator">3.5 + 4</gadget>
<output>7.5</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">7.5 / (1/5)</gadget>
<output>37.5</output>

<result>37.5</result>