设 甲速度为X km/h,那么乙速度为1.5X km/h
80 / x = (80 / 1.5x) + 20 / 60
x = 80 km/hлл�Ǻ�, �аﵽæ�ͺ����x-1/x=3��ôxƽ��+1/xƽ��ֵΪ��(x - 1/x) ƽ��= xƽ�� + (1/x) ƽ�� - 2 = 9 xƽ�� + (1/x) ƽ�� = 9 + 2 = 11