Xiao Ming and Xiao Dong set off from two places 1720 meters away from each other. Xiao Dong walked for 1 minute before Xiao Ming set off. Xiao Ming walks 80 meters per minute, and Xiao Dong walks 70 meters per minute. How long will it take for them to meet each other?
小明和小东分别从相距1720米的两地相向出发，小东先走1分钟后，小明才出发．小明每分钟走80米，小东每分钟走70米，他俩多长时间后能相遇？
<result>15</result>