A reservoir is equipped with three water inlet pipes A, B, and C. It takes only 45 minutes to open pipe A to fill up, pipe B to fill in 60 minutes, and pipe C to fill in 90 minutes. If the three pipes are opened at the same time, how many minutes can the whole pool be filled?
一个蓄水池，装有甲、乙、丙三个进水管，单开甲管45分钟注满，单开乙管60分钟注满，单开丙管90分钟注满．如果三管一齐开放，几分钟可以注满全池？
<gadget id="calculator">1 / 45</gadget>
<output>1/45 = around 0.022222</output>

<gadget id="calculator">1 / 60</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">1 / 90</gadget>
<output>1/90 = around 0.011111</output>

<gadget id="calculator">(1/45) + (1/60) + (1/90)</gadget>
<output>1/20 = around 0.05</output>

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

<result>20</result>