Lele went swimming in the swimming pool. He swam 4 back and forth in the swimming lane, and swam a total of 640 meters. How long is the swimming lane in this swimming pool?
乐乐去游泳池游泳，他在泳道上游了4个来回，共游了640米，这个游泳池的泳道有多长？
<result>50</result>