There are sixty or seventy students sitting in the classroom.
Tips
grammar
Chinese expresses 'approximately N or N+1' simply by stringing two adjacent numbers together: 三四 'three or four', 五六 'five or six', 六七十 'sixty to seventy', 两三百 'two or three hundred'. No connective like 'or' is needed — adjacency does the work.
usage
Frequently followed by 岁 (suì) for age estimates, 个 (gè) for rough counts of people or items, and 块 (kuài) for casual price ranges (六七十块 'sixty or seventy kuai').
