The "doctor's hat" is made of black cardboard, and the top is a square with a side length of 30 cm. Below is a bottomless cylinder with a diameter of 16 cm and a height of 10 cm; how many square decimeters of black cardboard are needed to make 20 such "doctoral hats"?
"博士帽"是用黑色卡纸做成的，上面是边长30厘米的正方形．下面是底面直径16厘米，高10厘米的无底无盖的圆柱；制作20顶这样的"博士帽"，至少需要黑色卡纸多少平方分米？
<gadget id="calculator">30 * 30</gadget>
<output>900</output>

<gadget id="calculator">3.14 * 16 * 10</gadget>
<output>502.4</output>

<gadget id="calculator">900 + 502.4</gadget>
<output>1_402.4</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1_402.4 * (1/5)</gadget>
<output>280.48</output>

<result>280.48</result>