For a pile of sand, it takes 6 o'clock to finish transporting car A, 8 o'clock to finish transporting car B, and 9 o'clock to finish transporting car C. After A and B have joined together for 3 hours at the same time, C also joins in. How long will it take to complete the shipment?
一堆沙子，甲车运完要6时，乙车运完要8时，丙车运完要9时．甲、乙同时合运3时后，丙也加入进来，还需多少时可运完？
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 / 8</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">(1/6) + (1/8)</gadget>
<output>7/24 = around 0.291667</output>

<gadget id="calculator">(7/24) * 3</gadget>
<output>7/8 = around 0.875</output>

<gadget id="calculator">1 - (7/8)</gadget>
<output>1/8 = around 0.125</output>

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

<gadget id="calculator">(1/6) + (1/8) + (1/9)</gadget>
<output>29/72 = around 0.402778</output>

<gadget id="calculator">(1/8) / (29/72)</gadget>
<output>9/29 = around 0.310345</output>

<result>9/29 = around 0.310345</result>