Sharing problem #3 from the European Girls’ Math Olympiad with kids

Yesterday I saw the great news that team USA won the European Girls’ Math Olympiad:

Flipping through the problems last night, problem #3 really caught my eye as one that math students might really enjoy because the solution is really cool. Here’s the problem:


This afternoon I thought it would be fun to talk through the problem with the boys. I have no expectation that they would be able to solve this problem – obviously! – but I really did think that a sketch of the solution would be really interesting to them.

I started by talking through the problem to make sure that they would understand it:

Once the boys understood the problem we dove into trying to solve it – where do you even begin – both boys said in the last video that the problem seemed impossible! Starting with some simple configurations with 2, 3, and 4 lines helped us see that the answer to the problem might be “no”.

To wrap up I showed the boys how you solve this problem via a coloring argument. The critical idea is that you can color the regions that are formed by the lines, with no two regions sharing a side having the same color – with just two colors. Once you have the coloring, there’s a fun little “aha” moment when you watch the path the snail takes . . . .

So, a seemingly impossible problem has a really pretty and really instructive solution. I think the coloring idea is something that middle school and high school kids who are interested in math will really enjoy seeing.


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