When is the relative velocity of two moving cars zero?

$A$ student is standing at a distance of $50 \, m$ from the bus. As soon as the bus begins its motion with an acceleration of $1 \, m/s^2$,the student starts running towards the bus with a uniform velocity $u$. Assuming the motion to be along a straight road,the minimum value of $u$,so that the student is able to catch the bus is.........$m/s$.

Two trains,one of length $100 \, m$ and another of length $125 \, m$,are moving in mutually opposite directions along parallel lines. They meet each other,each with a speed of $10 \, m/s$. If their accelerations are $0.3 \, m/s^2$ and $0.2 \, m/s^2$ respectively,then the time they take to pass each other will be ......... $s$.

$A$ person sitting in a moving train with his face towards the engine,throws a coin vertically upwards. The coin falls ahead of the person. The train:

$A$ particle is dropped from rest and another particle is thrown downward simultaneously with initial speed $u$. Then:

$A$ ball is dropped from the top of a building of height $80 \,m$. At the same instant,another ball is thrown upwards with a speed of $50 \,m/s$ from the bottom of the building. The time at which the balls will meet is ........ $s$.