Here are some rules for writing numbers.
For whole numbers under ten, write them out as words. For ten, 11, and twelve, discuss the matter with your copy editor, engaging in a long-running and frank exchange of typographical views that will, as always, end with at least one of the parties arrested for stuffing a body part into a toaster on the “bagel” setting that is itself stuffed into a composting pit, and might bring in some other parties who will discover they can not believe these other people are allowed to vote or hold sharp objects such as hula hoops. If the argument is not productive enough bring up the matter of zero and what results will surely end with arson. For numbers larger than twelve use digits, as they’re too tedious if given the chance to be words. Exceptions: googol googolplex either neither fimble flumble seizure leisure sixty-four caffeine.
When writing a string of numbers it’s important to alternate between digits and words for clarity, as for example in the famous aircraft being the Boeing “seven40seven” or the less famous aircraft the Boeing “7.thirty.7”. In addition to reading clarity the graphic design potential is powerful, and if you can’t imagine a trendy club writing its address this way you’ve failed graphic design class and probably can’t even recognize Futura when you see it, which doesn’t necessarily mean you’re a bad person but does mean we’ll have to have someone watching whenever you walk into the campus’s Fine Arts Library.
Percentages should never be referred to in print, as they make the reader suspect this is a word problem and the reader will go off to the bridge column. They may be used in PowerPoint slides only if the percent symbol is animated, rotating around one or two axes but not the third.
Negative numbers require special protection so as not to startle skittish readers. Besides being denoted with a minus sign they should be preceded by a man on foot waving a large red flag, and followed by another man ringing a bell. It is good practice to surround the number in parentheses, in case of spilling, and to be printed in red ink, lest the supply of red ink get noticeably too large. At that, you’ll want to have your copy reviewed by trained professional mathematicians. Do not rely on the untrained kind, as they will try to clean up spilled negative numbers by having them (the numbers) eaten by a goat. Trained professional mathematicians will call in something from accredited accounting ungulates.
Imaginary numbers may be written any old way you like, as the non-mathematical reader thinks you’re just making them up anyway, while the mathematically inclined roll their eyes and sigh knowingly whenever the subject comes up. Really, you probably don’t even have to do that much. Rewrite the sentence to avoid the whole subject, even if you have to change the essay’s subject from the history of polynomials into, oh, lumps of putty.
If you need to pluralize a number go wild and add an apostrophe before the s or es, as in: 7.thirty.7’es. In fact, nobody’s ever lived to regret adding apostrophes where they’re not needed, so, what the heck, toss in something so, like, this year is known as 201’4, or the population of the United States as 317′,84’2,’000. Apostrophes are also cool if you need to omit the part of the number that’s boring. Why not try writing the volume of your refrigerator in cubic inches as 14’82 and leave the reader to work out the omitted numbers for their fun and mental exercise, other than that if the reader finds out where you live they might jab you with an apostrophe in front of the toaster?
Know the difference between ordinal and cardinal numbers! If mixed they will fight until one is stuffed into a toaster and the other sneaks off to make long-distance calls on your land line. In cases of ambiguity remember that cardinal numbers are nearly invariably Rh-positive while ordinal numbers are afraid of bats, owing to the longstanding resentment of ordinal numbers for vampire novels after their manuscripts were rejected.
If you don’t like those rules, try some other ones. That’ll go well.