Xiao Cong uses a rope to measure the depth of a well. He puts one end of the rope into the bottom of the well. The length of the rope outside the wellhead is 9 meters. There are still 3 meters of rope, what is the depth of the well in meters?
小聪用一根绳子来测量一口井的深度，他把绳子的一端放入井底，井口外绳子长9米，小聪把这根绳子对折后，将一端入井底，这时在井口外的绳子还有3米，这口井的深度为多少米．
<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">9 - 6</gadget>
<output>3</output>

<result>3</result>