A train is traveling at 48 kmph It crosses another train having half of its length traveling in oppo...........

Explanation:

Speed of train 1 = 48 kmph
Let the length of train 1 = 2x meter
Speed of train 2 = 42 kmph
Length of train 2 = x meter (because it is half of train 1's length)
Distance = 2x + x = 3x
Relative speed= 48+42 = 90 kmph = (90*5/18)  m/s = 25 m/s
Time = 12 s
Distance/time = speed 

=3x/12 = 25 (25*12)/3 

Length of the first train = 2x = 200 meter
Time taken to cross the platform= 45 s
Speed of train 1 = 48 kmph = 480/36 = 40/3 m/s
Distance = 200 + y     [where y is the length of the platform] 

x =100m => 200+y = 45* 40/3
y = 400m

Hence, option 2 is the correct answer.