A 1500 m long train crosses a tree in 140 s, how much time will it take to pass a platform 800 m long? 

We can use the formula:

Speed = Distance/Time

Here, the distance covered by the train while crossing the tree is 1500 m and the time taken is 140 s. Therefore,

Speed = 1500 m / 140 s Speed = 10.71 m/s

Now, the train needs to cross the platform as well, which is 800 m long. So, the total distance that the train needs to cover is 1500 m + 800 m = 2300 m.

Using the speed calculated above, we can find the time taken to cover this distance:

Time = Distance/Speed Time = 2300 m / 10.71 m/s Time = 214.77 s (rounded to two decimal places)

Therefore, it will take the train approximately 214.77 seconds (or 3 minutes and 34 seconds) to pass the platform.