The key is the initial assumption "n is chosen uniformly at random in [0, 100]". This is rare in real life and few people can really understand it immediately.

The key is the initial assumption "n is chosen uniformly at random in [0, 100]". This is rare in real life and few people can really understand it immediately. In real life it's usually like "each ball can have certain chance to be red or green", in that case n is highly likely to be in the middle instead of equally distributed among [0,100].