Business Writing: Rules on Writing Numbers.
Spell out numbers above ten, unless writing the number would involve using more than two words.
Use words if the number can be written in two words of fewer. Remember that some words require a hyphen (twenty-six, thirty-nine). Some guides recommend that numbers up to nine should be written in words, and those over nine written using numerals.
Step I. Understand the principles of deliberate practice. Learning to write better can feel like an overwhelming task. But just like a chef learns how to fry an egg and how to fillet a fish—and just like a violinist practices a difficult passage over and over again—writers can practice specific writing techniques to improve their skills.
Whenever you are writing out a number that has three or more digits, there is no need to write the word and. However, you should use the word and to indicate or express decimal points. For example, Five hundred thirty-four dollars and forty-two cents.
Let the computer write the numbers. Type a number from 0 and 999,999,999 (Do not type the commas.).
These are better than numbers or letters as they draw your attention to each point without giving you extra information to take in. Apologising If you are replying to a tricky letter or a complaint, or are dealing with a difficult problem, put yourself in the reader's shoes.
Here’s something I know for sure: You can learn how to write better and add massive value to people’s lives—even if you’re not a “natural” writer. I know from direct experience. I’ve loved writing all my life, but at age 30, I was still an unpublished author struggling with the basics.