Brain Teasers & Mental Math
Two trains 100 miles apart drive toward each other, each at 25 mph. A fly leaves one train at 50 mph, flying back and forth between them until they collide. How far does the fly travel?
Model answer
100 miles.
Don't sum the infinite series of shrinking legs — just use time. The trains close at 25 + 25 = 50 mph, so they meet in 100 / 50 = 2 hours. The fly flies at 50 mph the whole time: 50 × 2 =…
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