Use three pipes A, B, and C to fill the reservoir with water. If only pipes A and C are turned on, the pool can be filled in 8 hours; if only pipes B and C are turned on, the pool can be filled in 10 hours. If the amount of water injected into pipe A per minute is twice the amount of water injected into pipe B per minute, the three water pipes A, B, and C are opened at the same time, how many hours will it take to fill the reservoir.
用甲、乙、丙三个水管往蓄水池中注水．若只开甲、丙两管，8小时可注满水池；若只开乙、丙两管，10小时可注满水池．如果甲管每分钟的注水量是乙管每分钟注水量的2倍，则同时打开甲、乙、丙三个水管，经多少小时可注满蓄水池．
<gadget id="calculator">1 / 8</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/8) - (1/10)</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">(1/8) + (1/40)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">1 / (3/20)</gadget>
<output>20/3 = around 6.666667</output>

<result>20/3 = around 6.666667</result>