1. Spell out single-digit whole numbers. Use numerals for numbers greater than nine.
I want five books.
I want 10 books.
2. Be consistent within a category. For example, if you choose numerals because one of the numbers is greater than nine, use numerals for all numbers in that category. If you choose to spell out numbers because one of the numbers is a single digit, spell out all numbers in that category.
If you have numbers in different categories, use numerals for one category and spell out the other.
My 10 dogs fought with their 2 cats.
My ten dogs fought with their two cats.
Given the economic crisis, if all 30 ELT students attend the four plays, then the 7 math students will be able to attend only two plays. (Students are represented with figures; plays are represented with words.)