5) 62 seconds
Let us first convert the speed given into m/s
(45 x 1000) ÷ 3600 = 12.5 m/s
Since the trains need to COMPLETELY emerge from the tunnel, the distance they need to cover must be the sum of the length of the tunnel and length of the train itself.
Since we need to find out the time taken for BOTH trains to emerge from the tunnel, we only need to find the time taken by the train that is going to take the longest to emerge.
Since the speed of both trains would be the same, we need to find out which train needs to cover the most distance.
Distance that must be covered = 80 + 615 = 695
Distance that must be covered = 40 + 120 + 615 = 775
Since the second train needs to cover the most distance, we just need to find the time taken by it to emerge from the tunnel.
Speed = 12.5 m/s
Time = 775 ÷ 12.5 = 62 seconds