In classical poetry, '二八' is often used to refer to a girl of sixteen.
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A classical multiplicative form: 二八 = 2×8 = 16. Compare 二九 (18), 三七 (21). The fixed phrase 二八佳人 (èr bā jiārén) — 'a beauty of sixteen' — is a stock image in Tang and Song poetry.
usage
Modern usage also references the '二八定律' (Pareto principle / 80-20 rule), where 二八 means 'two-vs-eight ratio' rather than the number 16.
