A, B, and C have to carry two piles of goods A and B, and the pile of goods in B is the pile of goods in A (5/4). If one person alone carries the pile of goods in A, A will finish the shipment in 20 hours, and B will finish the shipment in 24 hours. C completes the shipment in 30 hours, then A alone transports pile A, and B and C carry pile B. A few hours later, C goes to help A carry pile A, and the last two piles are shipped at the same time. How long does C help A carry?
甲、乙、丙三人要搬运A、B两堆货物，B堆货物是A堆货物的(5/4)，若单独一人运A堆货物，甲20小时运完，乙24小时运完，丙30小时运完，开始甲一人运A堆，乙、丙运B堆，几小时后，丙去帮甲运A堆，最后两堆货物同时运完，丙帮甲运了几小时？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">5 / 4</gadget>
<output>5/4 = around 1.25</output>

<gadget id="calculator">1 + (5/4)</gadget>
<output>9/4 = around 2.25</output>

<gadget id="calculator">1 / 24</gadget>
<output>1/24 = around 0.041667</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/20) + (1/24) + (1/30)</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">(9/4) / (1/8)</gadget>
<output>18</output>

<gadget id="calculator">(1/20) * 18</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">1 - (9/10)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/10) / (1/30)</gadget>
<output>3</output>

<result>3</result>