54km/h=15m/s
汽车在4s内行驶的距离为s₁=v₁t=15m/s×4s=60m
鸣笛声在4s内传播的总路程s₂=v₂t=340m/s×4s=1360m
司机鸣笛时离山的距离为s=

s1+s2

2

=

60m+1360m

2

=710m．
此时汽车离高山的距离：s′=710m-60m=650m．
答：司机听到鸣笛的回声时离高山的距离为650m．