Correct Option: A

Trains are running in opposite directions.
∴  Relative speed = 72 + 90
= 162 kmph

= 162 ×	5	= 45 m/sec
	18

Let the length of the first train be = x metre.
∴  Length of the second train

=	3	x meter.
	4

Now,

distance travelled in 3	1	seconds at 45 m/sec.
	2

= 45 ×	7	=	315	metre
	2		2

This distance is equal to sum of the lengths of trains.

∴  x +	3x	=	315
	4		2

⇒	4x + 3x	=	315
	4		2

⇒	7x	=	315
	4		2

⇒  x =	315	×	4	= 90
	2		7

Hence, the length of the first train = 90 metre.
Speed of first train = 72 kmph

= 72 ×	5	= 20 m/sec
	18

Time taken by the first train to cross the tunnel
= 3 minutes = 180 seconds
∴  Distance covered by it in 180 seconds
= 180 × 20 = 3600 metre
∴  Length of (first train + tunnel) = 3600 metre
∴  Length of tunnel
= 3600 – 90 = 3510 metre