30. The last four invariants problems (numbers 57-60) solved.

From a channel Lumen includes in full: Fields medallist Timothy Gowers works through olympiad and research-style problems live at the board, thinking aloud with all the false starts, plus lecture courses on topics such as additive combinatorics.

A 35-minute video from Timothy Gowers's catalogue; the title names its topic.

Video by Timothy Gowers, played through YouTube's embedded player.