In the sentence –
Auditors recommend an increase of the allowance for bad debts by ten %.
– should the percent sign be there or should the word percent be spelled out. Also should the number be written in a numeral format?
In general, it is good practice that the symbol that a number is associated with agrees with the way the number is written (in numeric or text form). For example, $3 instead of 3 dollars.
Note that this doesn't apply when the numbers are large, so it is perfectly fine to write 89.5 percent, as eighty-nine-and-a-half percent is very clunky.
This source puts it simply:
When writing percentages and money references, use the numeral with the percent or dollar sign.
Also, this answer on Academia.SE says "APA version 6 style manual has an entire section starting from 4.31 on how to present numbers in text" and provides a summary.
Therefore, you should write either 10% or ten percent - note that it is a common mistake for some to write per and cent separately. For some classical humour, check out this xkcd post.
What is incorrect is for the number to be written in text form but not for the symbol. That is, ten % or £ five is not good phrasing.
If you're writing out the number then write out 'percent', e.g. ten percent, otherwise use the sign, e.g. 10%.
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