Train A takes 6 hours more than train B to cover a distance of 600 km. If the speed of train A is doubled, it takes 2 hours less than train B. What is the speed of train B?

Let speed of B = v km/hr, speed of A = u km/hr. Distance = 600 km. Time A = 600/u, Time B = 600/v. First condition: 600/u = 600/v + 6. When A's speed doubles: 600/(2u) = 600/v - 2. Solving gives v = 60 km/hr. From equation: 600/v = 600/u - 6. Substituting v = 60: 10 = 600/u - 6, so u = 37.5. Verify: 600/37.5 = 16 hrs, 600/60 = 10 hrs. Difference is 6 hours ✓ When A doubles: 600/75 = 8 hrs, which is 2 hrs less than B ✓