The total area of land A, B, and C is 42 hectares. The area of land A is 12 hectares. The area of land B is 3/7 of the total area. How many hectares is the area of land C?
甲、乙、丙三块地的总面积是42公顷，甲地的面积是12公顷，乙地的面积是总面积的(3/7)，丙地的面积是多少公顷？
<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">42 * (3/7)</gadget>
<output>18</output>

<gadget id="calculator">42 - 12 - 18</gadget>
<output>12</output>

<result>12</result>