There is a rectangular parallelepiped steel with a length of 2 meters and a cross-section of a square with a side length of 5 cm. The steel weighs 7.8 kilograms per cubic decimeter. How many kilograms does this square steel weigh?
有一种长方体钢材，长2米，横截面是边长为5厘米的正方形，每立方分米钢重7.8千克，这根方钢材重多少千克？
<gadget id="calculator">5 / 10</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">2 * 10</gadget>
<output>20</output>

<gadget id="calculator">(1/2) * (1/2) * 20 * 7.8</gadget>
<output>39</output>

<result>39</result>