Suppose a Fire and Flying partner takes an Electric hit. Electric is ×1.6 against Fire? No — Fire resists Electric at ×0.6. But Electric is ×1.6 against Flying. Multiply the two: 0.6 × 1.6 = 0.96, so the hit lands roughly neutrally. That is the whole trick: single-type intuitions break on dual types, and the chart is the only reliable answer.
Run the same arithmetic on offence. A Water move against a Rock and Ground target is 1.6 × 1.6 = 2.56, capped down to ×2.4 — the best multiplier the game allows, and the reason a good matchup against a dual-type boss is worth reorganising your whole build for.