1设商品成本为x，原价为y，降价后为z(y-x)/y=0.25(1)(z-x)/z=0.1(2)由（1）得x=0.75y由（2）得x=0.9z0.75y=0.9zz/y=5/60.8333333……7.2分钟=0.12小时2设追上队伍用时x小时,则回返用时0.12-x小时，队伍长度为y4xy=6x4(0.12-x)6(0.12-x)=y解得x=0.1y=0.2队伍长0.2千米