Solve the all parts of given problem:
Problem: A person riding in the back of a pickup truck traveling at 70km/h on a straight, level road throws a ball with a speed of 20km/h relative to the truck in the direction of the truck's motion.
Part A: What is the velocity of the ball relative to a stationary observer by the side of the road?
Part B: What is the velocity of the ball relative to the driver of a car moving in the same direction as the truck at a speed of 100km/h?
Please provide step by step solution for above problem.