Once numerals have been used to describe a category, stick to numerals to complete the description of items in that category. Similarly, make sure to write the units in the same way. For example, don’t switch from kg to kilograms and back to kg in one paragraph.
Fixed: There were 2 bolts of 3 cm diameter, 12 bolts of 8 cm diameter, and 195 bolts of 20 cm diameter.
In the above example, the “two bolts” follows the generally understood rule of writing numbers of ten or less out in words. However, the counting of bolts is then done is words for “two” but numerals for “195”. This inconsistency can make reading confusing.
Correct example: The population grew from an initial 15,000 in 1960 to 21,000 by 1970 and 34,000 by 1980.
Correct example: Between 1,950 and 2,000 persons attended the concert.