A cuboid reservoir is 6.5 meters long and 3 meters wide when measured from the inside, and 39 cubic meters of water have been stored in the pool. It is known that the depth of the water is half of the depth of the pond. How deep is this reservoir in meters?
一个长方体蓄水池，从里面量长6.5米，宽3米，池中已蓄水39立方米．已知水的深度是池深的一半，这个蓄水池深多少米？
<gadget id="calculator">6.5 * 3</gadget>
<output>19.5</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">39 / 19.5 / (1/2)</gadget>
<output>4</output>

<result>4</result>