Let the length of the first train be x metres and the length of the bridge be y metres.

∴ The first train running with the speed of 90 km/h crosses the bridge by covering a distance of (x + y) meters in 36 seconds i.e. The first train crosses the bridge in 36 seconds running with the speed of 25 meters per second.

∴ x + y = 36 × 25 = 900

The second train crosses the bridge by covering a distance of [(x – 100) + y] meters with the speed of 45 km/h, i.e. with the speed of 12½ metres per second.

Since x + y = 900, therefore the second train crosses the bridge by covering a distance of 800 meters @ 12½ meters per second, i.e. the second train crosses the bridge in

= seconds.