Let's talk numbers. This is another topic that I file under the category of 'things I always find myself having to correct when editing other people's work'. (In case you're wondering, two other topics in this category are the misuse of capitals and double spacing between sentences.)
Let's dive straight in, bearing in mind that there are few absolute rules on this topic and most of the following are really questions of style.
1. Digits or letters?
When using numbers in your writing, smaller whole numbers are generally spelt out (i.e. written as words) while larger numbers are written in digits. So '1' is written as 'one'; '100' is written as 100. The style we use is to spell out single digit numbers (i.e. up to nine) and to use digits after that. Others will spell out up to twenty. Whichever way you go, the basic rule of professionalism applies, that is: be consistent.
2. The decimal exception
As usual, these things are never straightforward. When a smaller number includes a decimal point, it is always presented in digital form: 2.4, not two point four. The latter would simply be too hard to comprehend. A related exception is that it's better to write very large numbers in words rather than with all those zeros: 2.4 million, not 2,400,000.
3. Don't start a sentence with digits
This 'rule' is fairly universal: no matter what the number, it's bad form to start a sentence with digits. 97 editors out of 100 would agree. No. Ninety-seven editors out of 100 would agree. That statistic is entirely made up, but you get the point.
If that becomes messy – "Four thousand, seven hundred and twenty-five school children skipped school last year" – rearrangement might be a better option: "The number of school children who skipped school was 4725".
4. 'Per cent' over '%'
Traditionally when presenting percentages it's been more formal to spell out 'per cent' rather than use the symbol. So we would write 'approved by 97 per cent of editors', not 'approved by 97% of editors'.
This is not a hard and fast rule and to some extent the way you go will depend on the context. In a scientific report, for instance, where there are percentages aplenty, I would tend to use the '%' symbol with digits to reduce space as much as anything else.
(By the way, in Australia we use 'per cent' as in the UK; 'percent' as one word is an American spelling.)
5. Hyphenate your age ... sometimes
Hyphens and dashes are a topic for another day, however there is one common area of hyphen (mis)use relating to numbers, and that is the way people's ages are presented. It works like this: "The five-year-old boy sat next to the girl, who was six years old." Confusing, huh?
Technically this is an issue about compound adjectives that are adverbial phrases ... but let's not go there. If the 'five-year-old' bit immediately precedes the 'boy' bit, hyphenate it. Otherwise don't.
There's plenty more to this caper and a quick search of the web will bring up many more rules and guidelines. But follow just this list and you'll make your editor's life easier.