Ok here is an approach everyone can follow.
Speed (velocity) is miles/hour or distance / time.
--> Speed = distance / time
--> Distance = speed * time
--> Time = speed * distance
Initial speed is 40 meters / second.
Final speed is 0 meters / second
Acceleration is defined as a rate or in distance per second per second.
--> Acceleration = meters / second / second (m/(s^s))
Thus the equation is:
Speed initial - acceleration*(time) = Speed final
(m/s) - (m/s/s)*(s) = m/s <-- units check out
40m/s - 2m/s/s*(s) = 0m/s (note that s is unknown)
Solving we get S = 20seconds (deceleration time)
It will then stop within 20seconds, the 30s question is irrelevant.
Thus, to check the final distance:
Distance = speed * time OR
Distance = Initial speed*time - (acceleration*time*time)/2 (use calculus to prove this...)
(meters) = (meters/second)*seconds - (meters/second/second)*seconds*seconds)
--> Distance = (40m/s)*20seconds - (2m/s/s*20*20)/2
= 800meters - 400meters = 400meters distance.
