Obviously walk around a round lake with a radius of 15m, the speed of the first half circle is 1 m/s, and the second half circle is 3 m/s, then name the average speed around the lake in meters/s.
明明绕着一个半径为15m的圆湖散步，前半圆的速度是1米/秒，后半圆是3米/秒，则命名绕湖一圈的平均速度是多少米/秒．
<result>1.5</result>