In A-or-B equals in-A plus in-B minus in-both — the overlap formula. In this 1-2-1, after the crosses, the middle 2 window exactly equals the two 1 windows combined. Plug in: 2 = 1 + 1 − overlap, so the overlap is zero — the yellow cells watched by both are empty: ×. School inclusion-exclusion, straight onto the board.

In the app, one new tip arrives for each day you play.

▶ Try 50 rounds in your browser A free demo, no install. All 2,000 rounds are in the app.