It takes Xiao Ming 30 seconds to go up the stairs on the escalator. If it takes 12 seconds to walk up the stairs while taking the elevator, ask: How many seconds does it take to go up the stairs on foot when the escalator is not moving?
小明乘电动扶梯上楼需30秒，如果在乘电梯的同时向上走需12秒，问：电动扶梯不动时徒步上楼需几秒？
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/12) - (1/30)</gadget>
<output>1/20 = around 0.05</output>

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

<result>20</result>