The rule for using hyphens with numbers is fairly straight forward: if the number and word combine to be a compound adjective (describes a noun), then use a hyphen.
The 6-foot tall man ran the race in 10 s.
However, it is important to note that you do not use a hyphen when using an abbreviated unit of measure, even if it describes a noun.
The 6 ft tall man ran the race in 10 s.
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