If I had only one day left to live, I would live it in my tenth-grade math class, because it would seem like infinity. If numbers do the same to you, fear not: you can master them by knowing the rules.
Plurals. Spelled-out numbers form their plurals as other nouns do.
- The contestants were in their twenties and thirties.
- The family was at sixes and sevens. (in a state of disorder)
- Among the test scores were two 240s and three 238s.
- Competitive water skiing began in the 1950s and has flourished since the 1980s.
Comma between digits. In most numerals of one thousand or more, commas are used between groups of three digits, counting from the right. However, in scientific writing, commas are often omitted in four-digit numbers. No commas are used in page numbers, addresses, and years (though years of five digits or more do include the comma).
- Directions to the lake can be found on page 1535 of the user's manual.
- The ski-boat factory was originally located at 93301 Indian River Drive.
- Human artifacts dating from between 35,000 BC and 5000 BC have been found there.
Space between digits. In the International System of Units, half-spaces rather than commas are used to mark off groups of three digits, both to the left and to the right of the decimal point. In numbers of only four digits either to the left or the right of the decimal point, no space is used (except in table columns with numbers having five or more digits). This system is far more common in Europe than in the United States.
- 1 432 988
- 0.000 844
- 2301.2219 (four-digit numbers require no space)
Source: Chicago Manual of Style