A cylindrical steel with a length of 12 decimeters is cut into two sections, and the sum of the surface areas of the two sections increases by 7.8 square decimeters. What is the original volume of this bar in cubic decimeters?
一根长12分米的圆柱形钢材，截成两段后，两段表面积的和比原来增加7.8平方分米．这根钢材原来的体积是多少立方分米？
<gadget id="calculator">7.8 / 2</gadget>
<output>3.9</output>

<gadget id="calculator">3.9 * 12</gadget>
<output>46.8</output>

<result>46.8</result>