A pile of sand, the first time transported away (4/7), the second transported away the rest (1/9), the first time transported 11 tons more than the second time, how many tons is this pile of sand?
一堆沙子，第一次运走(4/7)，第二次运走余下的(1/9)，第一次比第二次多运11吨，这堆沙子有多少吨？
<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">1 - (4/7)</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">(3/7) * (1/9)</gadget>
<output>1/21 = around 0.047619</output>

<gadget id="calculator">(4/7) - (1/21)</gadget>
<output>11/21 = around 0.52381</output>

<gadget id="calculator">11 / (11/21)</gadget>
<output>21</output>

<result>21</result>