Q:
      
      
         
            
A train is traveling at 48 kmph . It crosses another train having half of its length , traveling in opposite direction at 42 kmph, in 12 seconds. It also passes a railway platform in 45 seconds. What is the length of the platform ?
         
       
      
      
      
          
      
      
          Answer & Explanation
         Answer: B) 400         
         
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
 (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
y = 400m
       
      
      
      
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