The ages of A, B, C, and D this year are 21, 13, 11, and 16 years old respectively. How many years ago, the sum of the ages of A and B was twice the sum of the ages of C and D.
甲、乙、丙、丁今年的年龄分别是21、13、11、16岁，多少年前，甲、乙的岁数和是丙、丁的岁数和的2倍．
<gadget id="calculator">2 * 11</gadget>
<output>22</output>

<gadget id="calculator">2 * 16</gadget>
<output>32</output>

<gadget id="calculator">32 - 21 - 13</gadget>
<output>-2</output>

<gadget id="calculator">22 + (-2)</gadget>
<output>20</output>

<gadget id="calculator">2 - 1 - 1</gadget>
<output>0</output>

<gadget id="calculator">2 + 0</gadget>
<output>2</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<result>10</result>