There is a pool in Dejiang People's Park. The whole pool can be drained in 15 hours with drainpipe A, and the whole pool can be drained with drainpipe B in 20 hours. Now, with both drainpipes A and B at the same time, half of the pool can be drained in a few hours. ?
德江人民公园有一水池，用甲排水管15小时可排完一整池水，用乙排水管20小时可排完一整池水，现在同时使用甲、乙两个排水管，几小时可以排完半池水？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/15) + (1/20)</gadget>
<output>7/60 = around 0.116667</output>

<gadget id="calculator">(1/2) / (7/60)</gadget>
<output>30/7 = around 4.285714</output>

<result>30/7 = around 4.285714</result>