A duck and a goose weigh a total of 12 kg. If you know that a chicken and a duck weigh 7 kilograms together; a goose and a chicken weigh 9 kilograms together, then how much is a duck in kilograms?
一只鸭、一只鹅共重12千克．如果知道一只鸡和一只鸭共重7千克；一只鹅和一只鸡共重9千克，那么一只鸭是多少千克．
<gadget id="calculator">12 + 7 + 9</gadget>
<output>28</output>

<gadget id="calculator">28 / 2</gadget>
<output>14</output>

<gadget id="calculator">14 - 9</gadget>
<output>5</output>

<result>5</result>