设静水速为每小时xkm,则顺水速为(x+2.5),逆水速为(x-2.5).
4.5(x+2.5)=6(x-2.5).
4.5x+11.25=6x-15
26.26=1.5x
x=17.5
答：静水速为每小时17.5km.