In ordinary writing in which numbers do not occur frequently, spell all amounts from one to one hundred inclusive, and also round numbers.
1. There are eighty-three people in our employ.
2. The paper containedone hundred signatures.
Note, however, that in writing numbers of five or more digits in the thousands it is preferable to use figures whenever it would be awkward to spell them. For example,you should express twenty-one thousand in words, but 21,512 in figures.
The following special rules will be found helpful:
1. Spell numbers or signs beginning a sentence or immediately following a colon. In such cases, if you prefer, you may change the arrangement of the sentence.
1. Seventy-five dollars is too much.
2. Plus and minus sometimes occur together in algebra. (Not 4- and .)
3. Fifty or sixty orders were received. (Not Fifty or 60 orders were received.”)
2. When two numbers occur together, express one of them in words thus :
1. Ten 5-room cottages.
2. Seven 8-inch guns.
3. 150 Fifth Avenue.
3. Express in words the time of day. In time-tables, however, figures are used.
1. Eight-thirty.
2. Ten o’clock.
If associated with several other sums, use figures.
Fifty rupees was the admission fee.
5. Express ages in words.
The secretary was fifty years of age.

In writing that involves the frequent enumeration of weights, measures, distances, sums of monty, dates, degrees, percentages, proportions, stocks and bonds, etc., figures are much more freely used.

This applies especially to adjustment letters, collection letters, order letters, contracts, and to statistics of any kind. The following special rules should prove helpful:
1. Express dates in figures, but spell the month.
June 25, 1921. (Not 6/25/21)
2. Express in figures street numbers and house numbers.
1014 N. 10 Street.

4. Express per cents, degrees, and ratios in figures.
1.7 per cent interest.
2. Longitude 70 05′ 08” E.
3. The ratio is 3 to 5.

5. Express in figures a list of articles such as may occur in inquiries or statements about them.
Please send 3 doz. not. 36 shirts.

As I said, do not get overwhelmed with this grammar part. But if you are already confident about it, then be sure to read other articles in this series.

Rajesh Gurule