Length of slower train
The person sitting in the faster train passes the slower train. This means that the speed differential when they go in same direction will represent the length of the slower train.

54 - 36 = 18km/hr --> this is the length of the slower train and it also equals to 30 seconds.

To find the length of the slower train in meters: 30 sec. = 1/2min = 1/120hr

Then, 18000m/hr / 1/120hr = 150meters

Length of the faster train
We know that the total length of the two train is found by adding up their respective speed.

54 + 36 = 90km/hr --> this is the length of both train and it is also equal to 10 seconds.

To find the total length of both trains in meters: 10 sec. = 1/6min = 1/360hr

Then, 90000m/hr / 1/360hr = 250meters

We know that length of slower train is 150meters

Therefore, length of faster train is 100 meters

OA is?

This is a good rate problem. Keep it going rakesh

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Best Regards,

Paul