51 problems carry newly within reach

This is the most optimistic tag in the collection. The problem may be old — sometimes very old — but something recent changed the odds: a new instrument, cheap computation, a machine-learning method, a dataset that finally exists. Problems wearing this tag are where fresh effort pays off most, because the historical record of failure no longer predicts the future. The people who failed before weren't less capable; they were missing a tool that now exists.