Stacking two adjacent numbers expresses an approximate range —
四五十 = '40-50.' This pattern works across magnitudes:
三四个 (3-4),
七八岁 (7-8 years old),
两三天 (2-3 days),
五六百 (500-600),
千八百 (around 1000-1800). Always pair adjacent digits; you wouldn't say
二六十.
