A train running with a speed of 108 km/h crosses a bridge in 5 seconds. Another train, 70 m shorter, crosses the same bridge at 72 km/h. Find the time taken by the second train to cross the bridge.

Let the length of the first train be L m.

Length of the second train = (L - 70) m

And length of the bridge = x m 

Speed of first train (V) = 108 km/h = 108 * 5/18 = 30 m/s

Speed of second train (v) = 72 km/h = 72 * 5/18 = 20 m/s

Now,

t = (L + x)/V 

5 = (L + x)/30 

L + x = 150 -------------------------(1)

Similarly,

t = (L - 70 + x)/20 ----------------(2) 

Now, putting the value of (L + x) in (2), we get

t = (150 - 70)/20 

t = 4 seconds