If you are writing a number sentence based on a given word problem do you include the clarrifying words or just the numbers? There are 12 students in class. If they are divided into 3 groups, how many children will be in each group?
English teachers prefer that you write out the numbers (i.e. twelve; three). It always annoys me when magazines like The New Yorker write out long numbers as words. That is the style that their editors insists upon. In the case of a journal publication, it is normal to write such numbers as numbers.
Being an engineer, I prefer that numbers be written with numbers rather thah letters. My personal preferences don't matter much, however.