A circular piece of aluminum has an outer radius of 4 decimeters and an inner radius of 3 decimeters. If each square decimeter of aluminum weighs 20 kilograms, how much does this piece of aluminum weigh?
一块圆环形的铝片，外半径是4分米，内半径是3分米．如果每平方分米的铝重20千克，这块铝片重多少？
<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

<gadget id="calculator">3 ** 2</gadget>
<output>9</output>

<gadget id="calculator">16 - 9</gadget>
<output>7</output>

<gadget id="calculator">3.14 * 7 * 20</gadget>
<output>439.6</output>

<result>439.6</result>