A cylindrical pool made of bricks has a wall thickness of 40 cm and a diameter of 18 meters when measured from the outside. What is the diameter in meters when measured from the inside?
用砖砌成的圆柱形水池，池壁厚40厘米，从外面量直径是18米，从里面量直径是多少米．
<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(2/5) * 2</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">18 - (4/5)</gadget>
<output>86/5 = around 17.2</output>

<result>86/5 = around 17.2</result>