A swimming pool can hold 840 tons of water. The swimming pool is equipped with a water inlet pipe and an outlet pipe. If the water inlet pipe is only opened for 8 hours, the empty pool can be filled, and if the water outlet pipe is only opened for 7 hours, the full pool can be emptied. Open, how many hours can the full pool be emptied?
一个游泳池可以容水840吨，游泳池装有一个进水管，一个出水管，若单开进水管8小时能把空池注满，若单开出水管7小时可以把满池水排空，两管一起开，几小时能把满池水排空？
<gadget id="calculator">840 / 7</gadget>
<output>120</output>

<gadget id="calculator">840 / 8</gadget>
<output>105</output>

<gadget id="calculator">120 - 105</gadget>
<output>15</output>

<gadget id="calculator">840 / 15</gadget>
<output>56</output>

<result>56</result>