Since the trains are going in opposite directions, their speed in relation to each other is the total of their two speeds. 63 + 45 = 108 kmh.

We'll need to do some conversion here. 1 kilometer = 1000 meters, and 1 hr = 3600 seconds.

108 kilometers = 108,000 meters
1 hour = 3600 seconds

108 km/h = 108,000m/3600s = 30 meters per second

So in 15 seconds, the trains cover 30 x 15 = 450 meters.

This answers only the total length:
LTa (Length of Train A) + LTb (Length of Train B) = 450 meters

From the second Part of the question

Since the man could see the other train getting passed in 40 sec.

LTa = (63-45)* (40/3600) = 200meters

So Ltb = 450 - 200 = 250 meters