t_总=30s+60s+30s=120s，s_总=900m+300m+900m+300m=2400m，v=

s总

t总

=

2400m

120s

=20m/s；20m/s=20×3.6km/h=72km/h．答：火车在进站、停站、出站的过程中平回均速度答为20m/s，合72km/h．