Assume we have two trains moving on the same track. These trains are approaching each other head-on meaning that Train A is moving towards Train B & vice versa. The track these trains are on has a distance of 120 km. Train A has a speed of 26.6 km/hour and Train B has a speed of 19.7 km/hour.
Now on top of Train A, there is a little bird that is flying with a speed of 33.4 km. The bird flies from Train A to Train B and back. He keeps doing this until both trains approach each other.
The question is: How much distance has the bird flown once both trains approached each other?
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The bird flies a distance of 90 km back and forth until the trains reach each other.
Step 1: Sum up the speed of both trains and divide the total distance of 120 km by the summed up speed.
120 km/44.3 km=2.7 hours which is about 2 hours and 40 minutes. Feel free to round up and estimate here.
Step 2: Now that we know how long it will take before the two trains reach each other, we can calculate the distance the little bird has flown.
2.7 hours*33.4 km=90 km.
Conclusion: In the timeframe of 2.7 hours the bird flies a distance of 90 km back and forth until the trains reach each other.