Use the rope to log the depth of the well, and if you fold it in four, there will be 9 meters left; after cutting the rope to 18 meters, and fold it in three, the remaining 12 meters will be 12 meters. How many meters is the depth of the well?
用绳子测井的深度，四折而入，则余9米；把绳子剪去18米后，三折而入，则余12米，井深多少米．
<gadget id="calculator">3 * 12</gadget>
<output>36</output>

<gadget id="calculator">4 * 9</gadget>
<output>36</output>

<gadget id="calculator">36 - 36</gadget>
<output>0</output>

<gadget id="calculator">18 + 0</gadget>
<output>18</output>

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

<gadget id="calculator">18 / 1</gadget>
<output>18</output>

<result>18</result>