pool has a number of pipes that fill each minute with equal amounts of water. Now open a number of water pipes, and after a predetermined time (1/4), double the number of open water pipes to fill the pool according to the predetermined time; Can fill the pool according to the scheduled time. Q: How many water pipes were opened at the beginning?
一水池有一些水管，它们每分钟的注水量都相等．现在打开若干根水管，经过预定时间的(1/4)，再把打开的水管增加两倍，就能按预定时间注满水池；如果开始时就打开15根水管，中途不增开水管，也能按预定时间注满水池．问：开始打开了几根水管？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/4) * (1/15)</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">3 * (1/15)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/4) * 3 * (1/15)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/5) - (1/20)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">(1/60) + (3/20)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 / (1/6)</gadget>
<output>6</output>

<result>6</result>