Mom bought back 21 eggs and duck eggs, of which duck eggs accounted for (3/7); later, mother bought a few more duck eggs, and then duck eggs accounted for (7/13) of the total number of eggs, and later mother bought a few more Duck eggs?
妈妈买回鸡蛋和鸭蛋共21个，其中鸭蛋占(3/7)；后来，妈妈又买回几个鸭蛋，这时鸭蛋占总蛋数的(7/13)，后来妈妈又买回来几个鸭蛋？
<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">1 - (3/7)</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">7 / 13</gadget>
<output>7/13 = around 0.538462</output>

<gadget id="calculator">1 - (7/13)</gadget>
<output>6/13 = around 0.461538</output>

<gadget id="calculator">(4/7) / (6/13)</gadget>
<output>26/21 = around 1.238095</output>

<gadget id="calculator">21 * (26/21)</gadget>
<output>26</output>

<gadget id="calculator">26 - 21</gadget>
<output>5</output>

<result>5</result>