A tunnel of 600 m length has two parallel rail tracks laid in it. Two trains of length 400 m and 200 m enter the tunnel simultaneously. After how much time from the instant the two trains entered the tunnel, will the tunnel be free of traffic? (Given the speeds of the two trains, of lengths 400 m and 200 m are 20 m/s and 10 m/s respectively)
Solution
The 400m long trains would cross the tunnel in $\frac{600+400}{20}=50$seconds.
The 200 m long train would cross the tunnel in $\frac{600+200}{100}=80$seconds.
Hence in 80 seconds, i.e. the maximum of the times taken by trains to cross the tunnel, the tunnel would be free of traffic.