A certain type of ring has an outer diameter of 58 mm and an inner diameter of 40 mm and a thickness of 1 mm. If one stacks enough rings on top of each other, it is possible to stand another ring vertically on top of the pile in such a way that the ring doesn’t
touch the ground. What is the minimum number of rings
you need to stack on top of each other so that the vertical ring just doesn’t touch the ground?
This was problem 9 on the South African Mathematics Olympiad (Junior Round 3 Paper 2012)