There is a square well with a wellhead area of 81 square decimeters and a depth of 4.5 meters. At this time, the wellhead is 5 decimeters away from the water surface in the well. It is known that the water weight per cubic meter is 1 ton. What is in the well at this time? How many tons of water?
有一口井口是正方形的井，井口面积是81平方分米，井的深度是4.5米，此时井口距离井里水面5分米，已知每立方米水重1吨，请问此时井里有水多少吨？
<gadget id="calculator">4.5 - 0.5</gadget>
<output>4</output>

<gadget id="calculator">0.9 * 0.9 * 4 * 1</gadget>
<output>3.24</output>

<result>3.24</result>