There are three water pipes installed on a pool, pipe A is the water inlet pipe, pipes B and C are the outlet pipes, among which pipe B can drain the full pool of water in 30 minutes, and pipe C can drain the full pool of water in 60 minutes. Now open pipe A first. When the water in the pool just overflows, it takes 28 minutes to open pipes B and C to drain the water; then when pipe A is full, then open pipe B instead of pipe C. How many minutes can the water be drained? Finished?
一个水池上装有三个水管，甲管为进水管，乙管和丙管为出水管，其中乙管30分钟可将满池水放完，丙管60分钟可将满池水放完．现在先打开甲管，当水池的水刚溢出时，打开乙、丙管用了28分钟放完；那么当打开甲管水满时，再打开乙管，而不开丙管，多少分钟可以将水放完？
<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

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

<gadget id="calculator">(1/30) + (1/60)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 28</gadget>
<output>1/28 = around 0.035714</output>

<gadget id="calculator">(1/20) - (1/28)</gadget>
<output>1/70 = around 0.014286</output>

<gadget id="calculator">(1/30) - (1/70)</gadget>
<output>2/105 = around 0.019048</output>

<gadget id="calculator">1 / (2/105)</gadget>
<output>105/2 = around 52.5</output>

<result>105/2 = around 52.5</result>