1. State the problem: A train 300 meters long runs at 90 km/hr and needs to cross a 600-meter-long platform. We want to find the time it takes to cross the platform. 2. Understand that the train must cover the length of the train plus the platform to completely cross it. So the total distance $$d = 300 + 600 = 900$$ meters. 3. Convert speed from km/hr to m/s for consistent units: $$90 \text{ km/hr} = 90 \times \frac{1000}{3600} = 25 \text{ m/s}$$. 4. Use the formula for time: $$t = \frac{d}{v} = \frac{900}{25} = 36$$ seconds. 5. Final answer: The train takes $$36$$ seconds to cross the platform.