The edge length of the big cube is twice the edge length of the small cube, and the volume of the big cube is 21 cubic decimeters larger than that of the small cube. What is the volume of the small cube?
大正方体棱长是小正方体棱长的2倍，大正方体体积比小正方体体积多21立方分米．小正方体的体积是多少？
<gadget id="calculator">8 - 1</gadget>
<output>7</output>

<gadget id="calculator">21 / 7</gadget>
<output>3</output>

<result>3</result>