If three water pipes A, B, and C are pouring water into a pond at the same time, it can be filled in half an hour; it can be filled in 1 hour with only two pipes A and B open; it can be filled in 45 minutes with only two pipes B and C open. How many hours does it take to fill a pool of water with pipe B alone?
如果甲、乙、丙三根水管同时向一个池子注水，半个小时可注满；只开甲、乙两管1个小时注满；只开乙、丙两管45分可注满．单开乙管注满一池水需要多少小时．
<gadget id="calculator">1 / 1</gadget>
<output>1</output>

<gadget id="calculator">45 / 60</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 / (3/4)</gadget>
<output>4/3 = around 1.333333</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">(4/3) - 2</gadget>
<output>-2/3 = around -0.666667</output>

<gadget id="calculator">1 + (-2/3)</gadget>
<output>1/3 = around 0.333333</output>

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

<result>3</result>