Use a rope to measure the depth of the well, fold the rope into three folds, and there is 2 feet outside the well; fold the rope into four folds, and the upper end of the rope is 1 foot below the mouth of the well, then the depth of the well is 2 feet.
用绳子测量井深，把绳子折成三折，井外余2尺；把绳子折成四折，绳子上端在井口下1尺，则井深多少尺．
<gadget id="calculator">1 * 4</gadget>
<output>4</output>

<gadget id="calculator">2 * 3</gadget>
<output>6</output>

<gadget id="calculator">4 + 6</gadget>
<output>10</output>

<gadget id="calculator">4 - 3</gadget>
<output>1</output>

<gadget id="calculator">10 / 1</gadget>
<output>10</output>

<result>10</result>