Unless they cannot be expressed in one or two words, use words for numbers that precede a compound modifier that includes another number.
She sold twenty 35-millimeter cameras in one day.
The prescription called for 120 25-milligram doses.
Note: You may use a combination of words and numerals for very large numbers.
6-8 million 2 billion to 2.9 billion 7 trillion
(From Write for Business, page 212, and Proofreader's Guide PDF, page 14)
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