A and B cooperated to clean up the 400-meter circular runway. They walked back to each other from the same place at the same time and worked separately. At first, A cleaned up faster than B (1/3). The working efficiency is twice as high as before. As a result, the cleaning work is completed after one hour from the beginning, and the runway cleaned by the two is the same length. How much time did B work after he exchanged tools?
甲乙两人合作清理400米的环行跑道，两人同时从同一地点背向而行各自进行工作，最初甲清理的速度比乙快(1/3)，中途乙曾用10分钟去换取工具，而后工作效率是原来的2倍，结果从开始算起，经过1小时，完成清理工作，并且两人清理的跑道一样长，问乙换取工具后又工作了多少时间？
<result>30</result>