A train is 1km long traveling at a constant speed. It takes 30s to enter a tunnel. After it is completely in the tunnel , it takes 90s to get out of the tunnel. How long is the tunnel?
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Since the train is 1km long and it took 30s to enter the tunnel, we know the train is traveling at 1km/30s. It will take 30s to get completely out of the tunnel from the time the engine begins to exit. Therefore, we know the train will travel an additional 60s (or 2k) inside the tunnel after completely entering, which would mean the tunnel is 3km long.