As you sit in your stopped car at a railroad crossing, a train travelling at 60 miles per hour takes twenty seconds to pass you. How many feet long is the train?

This classic brain teaser is a great example of how we can use familiar units of time and distance to solve a seemingly complex problem. From the perspective of a stationary observer, the length of the train is equivalent to the distance it travels in the time it takes to pass. The key is converting the train's speed into more manageable terms. A speed of 60 miles per hour is incredibly convenient, as it translates perfectly to one mile per minute.

Once we know the train travels one mile every minute, the rest of the calculation falls into place. The passing time is twenty seconds, which is exactly one-third of a minute. Therefore, in that time, the train must have traveled one-third of a mile. To get our final answer, we just need to convert this distance into feet. Since a mile contains 5,280 feet, we can find one-third of that distance by dividing 5,280 by three.

The result is a train that is 1,760 feet long. For context, that's precisely one-third of a mile. While this is a fairly long train, it's not unusual for freight trains in North America, which can often exceed a mile in length. So the next time you're stopped at a crossing, you can try to time the passing train and estimate its size for yourself