I made a rough depiction in paint so assume the red lines divide the big circle exactly by half and both circle's circumferences collide in a single point at the top of the vertical red line.
If you find a link to the geometrical property/ratio/whatever used to solved this please post it
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What's the answer, OP? Just wondering, haven't done any calculations myself as I'm too lazy to pull out a calculator, but it'll provide something for everyone to work towards. Also is this some kind of secret Marathon reference?