A 1050kg sports car is moving westbound at 20m/s on a level road when it collides with a 6320kg truck driving east on the same road at 20m/s. The two vehicles remain locked together after the collision. At what speed should the truck have been moving so that it and the car are both stopped in the collision?
a) 10 m/s
b) 20 m/s
c) 30 m/s
d) 40 m/s