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Generally spell out one through nine and use figures thereafter:
            six parts steam, 11 parts sweat
            six-to-four margin
            two to one against
            42 to 36 in favor

Odds and ratios can be written in all numerals, especially when the numeral 1 is used:
            1-in-20 chance
            10-to-1 return on investment

Spell out figures of speech such as fifty-fifty, unless the context makes numerals appropriate:
            Others thought he had a 1-in-10 chance of winning, though he considered his odds were 50-50.