设全程为2s,甲的速度为x+15,乙的速度为x,丙的速度为x-3甲走半程花费的时间s/(x+15),乙走半程花费的时间s/x,丙走半程花费的时间s/(x-3)s/x-s/(x+15)=4s/(x-3)-s/x=1s((x+15)-x)/x(x+15)=4s(x-(x-3))/x(x-3)=115/x(x+1...