Sets — O/L Mathematics

Every time a survey asks "how many students play cricket, how many play football, and how many play both?" you are really being asked a question about sets. The same idea runs quietly underneath probability, database searches, and any situation where two groups overlap and you must count people without counting the overlap twice. Get sets right at O/L and you gain a clean, reliable way to organise information — and you earn some of the most predictable marks on the paper.