There are 27 sparrows in the two trees, 4 sparrows fly from the first tree to the second tree, and 6 sparrows fly away from the second tree. At this time, the sparrows on the first tree are the second tree Twice as many sparrows as there are in the first tree.
两棵树上共有麻雀27只，第一棵树上飞到第二棵树上4只，又从第二棵树上飞走6只，这时第一棵树上的麻雀是第二棵树上的两倍，原来第一棵树有多少只麻雀．
<gadget id="calculator">27 - 6</gadget>
<output>21</output>

<gadget id="calculator">2 + 1</gadget>
<output>3</output>

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

<gadget id="calculator">7 * 2</gadget>
<output>14</output>

<gadget id="calculator">14 + 4</gadget>
<output>18</output>

<result>18</result>