A train 'M' crosses a pole in 20 seconds and a platform of length 400 metres in 36 seconds. Speed of another train 'N' of length 480 metres is 90 km/h. Both trains are standing at two extreme ends of a tunnel of length 220 metres. Both the trains start travelling towards each other simultaneously. Find the time taken by the front end of train 'M' to cross the rear end of train 'N'.