（1）∵1km=105cm，∴56km=56×105cm=5.6×106cm；（2）∵1min= 1 60 h，∴45min=45× 1 60 h=0.75h；（3）∵1km/h= 1 3.6 m/s，∴72km/h=72× 1 3.6 m/s=20m/s．故答案为：5.6×106；0.75；20．
