Let the total distance be d.
Time taken to cover (31)rd of total distance is t1=3v1d.
Time taken to cover (31)rd of total distance is t2=3v2d.
Time taken to cover (31)rd of total distance is t3=3v3d.
Average speed=TotaltimeTotaldistance
vavg=3v1d+3v2d+3v3dd
⇒vavg=v1v2+v2v3+v3v13(v1v2v3)
Here, v1=11ms−1, then, v3=33ms−1,v2=22ms−1
Vavg=(11×22)+(22×33)+(33×11)3×11×22×33=18ms−1