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Every $10$ minutes, two buses set out, one from each end.
How many buses will one bus meet on its way from one end to the other end?
See Crossing the Atlantic for a similar problem.Imagine you are suspending a cube from one vertex and allowing it to hang freely. What shape does the surface of the water make around the cube?
Given the nets of 4 cubes with the faces coloured in 4 colours, build a tower so that on each vertical wall no colour is repeated, that is all 4 colours appear.