Question

a train crosses a platform 90m long in 30s and a man standing on the platform in 15s . find the speed of the train

Answer 1

Simple. We know that 15 of the 30 seconds are due the train’s length, so the remaining 15 seconds are not. Therefore, a single part of the train passes the 90m platform in 15 seconds, implying its speed is 6m/s.