Geometric mean is the nth root of the product, i.e.

For example, if the market closes like this (cropped for brevity):

Would resolve YES as 20√(42570 × 5119 × 54 × 34195 × 1000 × 700 × 666 × 426 × 269 × 265 × 241 × 133 × 130 × 125 × 93 × 70 × 64 × 16 × 13 × 13) ≈ 267.

Resolves NO if exactly 225.

Geometric mean is more resistant than arithmetic mean from being influenced by a single large outlier.

I will trade on market but will not withdraw AMM liquidity at any point.