The Brain Teaser: Scroll Down for your Solution
Two trains, each two miles long, enter two one mile long tunnels that are two miles apart from one another on the same track. The trains enter the tunnels at exactly the same time. The first train is going 5 miles/hour, and the second train is going 10 miles/hour. What is the sum of the lengths of the two trains that will protrude from the tunnels at the exact moment that they collide, assuming that neither train changes its speed prior to collision? The trains are on the same track headed in opposite directions (i.e. directly toward one another).
2 2/3 miles. The trains are exactly 4 miles apart. Their combined speed is 15 miles-hour, so it will take them 16 minutes to collide. The first train will have traveled 1 1/3 miles, so 1/3 mile of it will be out of the tunnel in front, 2/3 mile of it will be out in the back on collision. The other mile of it will be in the tunnel. The second train will have traveled 2 2/3 miles, so only 1/3 mile of it will still be in the tunnel, so 1 2/3 miles of it will be out of the tunnel.