Open the drain pipe of a pool, it can be drained in (1/4) hour, if only the water inlet pipe is opened (1/2) hour, it can be filled, if the two pipes are opened at the same time, how many hours can the full pool be drained.
一水池打开排水管，(1/4)小时可排完，如果只打开进水管(1/2)小时可放满，如果两管同时打开多少小时可把满池水放完．
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

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

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / (1/2)</gadget>
<output>2</output>

<gadget id="calculator">4 - 2</gadget>
<output>2</output>

<result>1/2 = around 0.5</result>