The distance that the train needs to cover to cross the platform is the sum of its own length and the length of the platform, i.e., 120m + 230m = 350m.

First, we need to convert the speed of the train from km/hr to m/s as the distance is given in meters.

90 km/hr = (90 x 1000) / 3600 m/s = 25 m/s

Now we can use the formula:

time = distance / speed

So,

time taken to cross the platform = (distance covered by train + platform length) / speed of the train = 350m / 25m/s = 14 seconds

Therefore, it will take 14 seconds for the train to cross a platform 230 m long at a speed of 90 km/hr.