About forty or fifty people came to the classroom today.
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grammar
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). Always pair adjacent digits; you wouldn't say 二六十.
