Every evening, Xiaohong’s grandfather goes for a walk by the river. There are 35 willow trees evenly planted on both sides of the river, and there are 35 willow trees on each side. It takes 3 minutes for grandpa to go from the first tree to the seventh tree, and then he walks at the same speed. The last tree immediately returned to the first tree along the original road. How long did it take grandpa to walk along the river?
每天傍晚小红的爷爷都要到河边散步河道两岸均匀地栽着柳树，每边各有35棵，爷爷从第一棵起走到第七棵共用了3分钟接着也用同样的速度走到最后一棵，又立即沿原路返回了第一棵．爷爷沿河散步共用了多长时间？
<result>34</result>