There are two trains, red and purple, moving to each other with velocities v1 and v2 respectively. The distance between them is D.
A bird is sitting on the top of a bar near red train. The bird flies with v3 velocity where v3 > v1 and v3 > v2. The bird has to fly from red to purple train and fly back from purple to red.
The question is: How many times the bird can fly in between these two moving trains until the trains reach to each other? (see the image)
(I heard that this problem is associated to Gauss, since he couldn't perfectly solve this. Is that true?)