Two trains are on the same track a distance 100 km apart heading towards one another, each at a speed of 50 km/h. A fly starting out at the front of one train, flies towards the other at a speed of 75 km/h. Upon reaching the other train, the fly turns around and continues towards the first train. How many kilometers does the fly travel before getting squashed in the collision of the two trains?math - Reiny, Sunday, December 7, 2014 at 9:36pmSo the distance between the two trains is changing at a rate of 100 km/h, (the sum of their speeds)
so it will take them 100/100 or 1 hour to collide.
In the meantime, the fly is merrily flying back and forth at 75 km/h, so in the 1 hour it would have simply traveled 75 km.
Notice we ignored the fact that the fly has to decelerate and accelerate at each turn.