The correct option is C 25 ms−1
Using first equation of motion
v=u+at
40=(20)a1 (∵u=0)
∴a1=2 m/s2
⇒S1=12a1t2
∴S1=400 m
Thereafter it travels with constant speed of 40 m/s, hence distance travelled will be,
⇒S2=40×20=800 m
Further it travels with uniform retardation until final velocity becomes 0 Putting t=40 s,u=40 m/s and v=0 in first equation of motion, we get
0=40+(a2)t
−40=(40)a2
∴a2=−1 m/s2
∴ retardation = 1 m/s2
Putting a=−1 m/s2 in third equation of motion, we get
02−402=2(−1)S3
⇒S3=800 m
Now average velocity =Total displacementtime
=400+800+80020+20+40=25 m/s