There are two buckets, big and small, which hold 20 kilograms of water in total. After pouring out the same amount of water from both buckets, there are 3 kilograms left in the small bucket and 9 kilograms left in the large bucket. How many kilograms are there in the big bucket?
有大小两个水桶，一共装水20千克，两个水桶都倒出同样多的水后，小水桶还剩3千克，大水桶还剩9千克，原来大桶有多少千克？
<gadget id="calculator">3 + 9</gadget>
<output>12</output>

<gadget id="calculator">20 - 12</gadget>
<output>8</output>

<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<gadget id="calculator">4 + 9</gadget>
<output>13</output>

<result>13</result>