Normally, for numbers used in adjectival expressions, follow the rule given in 5.01 Introduction, i.e. write out those from one to nine and use numerals for the rest:
If the unit is represented by an abbreviation or symbol, use numerals (see 1.23 The International System of Units (SI)):
or
Do not use a hyphen between a numeral and a non-letter symbol:
When a number immediately precedes a compound modifier containing another number, spell out the first or the smaller number:
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