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Except for a few basic rules, spelling out numbers is largely a matter of writers’ preference, consistency is the key. The Associated Press Stylebook recommends spelling out the numbers zero through nine and using numerals thereafter. This is a complex topic, with many exceptions.
Rule 1. Spell out all numbers beginning a sentence.

Rule 2. Hyphenate all compound numbers from twenty-one through ninety-nine.

Rule 3. Hyphenate all written-out fractions.

Rule 4. With figures of four or more digits, use commas. Count three spaces to the left to place the first comma. Continue placing commas after every three digits. Important: do not include decimal points when doing the counting. $1,345,234.45

Rule 5. It is not necessary to use a decimal point or a dollar sign when writing out sums of less than a dollar.

Rule 6. Do not add the word “dollars” to figures preceded by a dollar sign.

Rule 7. For clarity, use noon and midnight rather than 12:00 PM and 12:00 AM.

Rule 8. Using numerals for the time of day has become widely accepted.

The flight leaves at 6:22 a.m.
Please arrive by 12:30 sharp.
However, some writers prefer to spell out the time, particularly when using o’clock.
She takes the four thirty-five train.
The baby wakes up at five o’clock in the morning.

Rule 9. Quantities less than 10 should be spelt out in full. She has one husband, three children, nine goats and 10 chickens.