It takes Xiao Ming 15 seconds to go up the escalator. If it takes 10 seconds to walk up the escalator while taking the escalator, 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15秒，如果在乘电梯的同时向上走需10秒，问：电动扶梯不动时徒步上楼需几秒？
<result>30</result>