There is a crack at the bottom of a cistern, and there are three water inlet pipes on it. When the tank is empty, if there are only two tubes A and B, it will take 12 hours to fill. If there are two tubes A and C, it will take 10 hours to fill up, and if only tube A is opened, it will take 15 hours to fill up. If the crack is blocked and only two pipes of ethylene and propylene are opened, how many hours will it take to fill the empty pool?
一个蓄水池的底部有一条裂缝，上面右甲乙丙三个进水管．空池的时候，如果只有甲乙两管，那么需要12小时才能灌满．如果甲丙两管，那么需要10小时可以灌满，如果只开甲管需要15小时灌满．如果把裂缝堵住以后，只开乙丙两管，需要多少小时，可以将空池灌满？
<gadget id="calculator">1 / 60</gadget>
<output>1/60 = around 0.016667</output>

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

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/10) - (1/15)</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/60) + (1/30)</gadget>
<output>1/20 = around 0.05</output>

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

<result>20</result>