Measure the water depth with a rope folded in three. The length of the rope above the water surface is 13 meters. If the rope is folded in half, the length of the part above the water surface is 3 meters. What is the depth of the water?
用绳子三折量水深，水面以上部分绳长13米，如果绳子五折量，则水面以上部分长3米，那么水深是多少．
<gadget id="calculator">13 * 3</gadget>
<output>39</output>

<gadget id="calculator">3 * 5</gadget>
<output>15</output>

<gadget id="calculator">39 - 15</gadget>
<output>24</output>

<gadget id="calculator">5 - 3</gadget>
<output>2</output>

<gadget id="calculator">24 / 2</gadget>
<output>12</output>

<result>12</result>