The minimum constant deceleration will have train A travelling at 30 m/s when it is at the same position as train B.

Let the distance travelled by train B at time be . Then

.... (1)

Data for Train A:

m

u = 80 m/s

v = 30 m/s

s

Use the usual formula :

.... (2)

Solve equations (1) and (2) simultaneously for .

So for Train A you now have:

u = 80 m/s

v = 30 m/s

a = ?

Use an appropriate formula to solve for a.