ANS:A - 45 m
	
		
			| 2 kmph = | 
			  | 
			2 x | 
			5 | 
			  | 
			m/sec = | 
			5 | 
			m/sec. | 
		
		
			| 18 | 
			9 | 
		
	
 
 
	
		
			| 4 kmph = | 
			  | 
			4 x | 
			5 | 
			  | 
			m/sec = | 
			10 | 
			m/sec. | 
		
		
			| 18 | 
			9 | 
		
	
 
Let the length of the train be 
x metres and its speed by 
y m/sec.
 
 

 9
y - 5 = 
x and 10(9
y - 10) = 9
x

 9
y - 
x = 5 and 90
y - 9
x = 100.
On solving, we get: 
x = 50.

 Length of the train is 50 m.