There are two water inlet pipes on a water tank, the empty water tank can be filled with water in 15 minutes by only opening the No. 1 water pipe, and the empty water tank can be filled with water by only opening the No. 2 water pipe for 10 minutes. If no more than 8 minutes are required to fill an empty tank with water, how long should both water pipes be open at the same time?
有一个水箱上装有两个进水管，单开1号水管15分钟可以把空水箱注满水，单开2号水管10分钟可以把空水箱注满水．如果要求不超过8分钟把空水箱注满水，那么两个水管至少应同时打开多长时间？
<gadget id="calculator">4 / 3</gadget>
<output>4/3 = around 1.333333</output>

<gadget id="calculator">(4/3) - 1</gadget>
<output>1/3 = around 0.333333</output>

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

<gadget id="calculator">8 - 5</gadget>
<output>3</output>

<result>3</result>