I’ve yet to find a pupil that can solve the problem below but what is lovely about this problem is that most pupils can access it and have some ideas about how to solve it. It hooks them but provides a real challenge to solve it. There are many methods you can use to solve the problem and it is lovely to see some pupils using algebra, some using speed = distance / time, some using trial and error, some using speed-time graphs, some using inequalities etc.
The solution is that one train is twice as fast as the other. Here’s the algebraic proof that I came up with: