A cistern is equipped with two water inlet taps. If only the first water inlet tap is opened, it takes 5 hours to fill the pool, and only the second water inlet tap is opened, and it takes 7 hours to fill the water tank. If both water inlet taps are opened at the same time, the water tank can be filled in a few hours ?
一个蓄水池装有两个进水龙头，如果只打开甲进水龙头要5小时注满水池，只打开乙进水龙头要7小时注满水池，如果两个进水龙头同时打开，几小时可以注满水池？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">(1/5) + (1/7)</gadget>
<output>12/35 = around 0.342857</output>

<gadget id="calculator">1 / (12/35)</gadget>
<output>35/12 = around 2.916667</output>

<result>35/12 = around 2.916667</result>