I'm preparing for a competitive exam and often get stuck on train problems where two trains are moving in the same direction. How do I correctly calculate the time it takes for a faster train to overtake a slower one, especially when they have different lengths?

I've been practicing problems from a book, and when a faster train needs to overtake a slower train moving in the same direction, I'm unsure about handling the distances. For instance, if Train A is 200 meters long moving at 90 km/h, and Train B is 150 meters long moving at 60 km/h, I know I should use relative speed, but I keep getting confused about whether to subtract the speeds or add them, and how the lengths factor in. I've tried using formulas like time = (sum of lengths) / (difference in speeds), but my answers don't always match the solutions provided.