This is a good site: http://www.grammarbook.com/numbers/numbers.asp
Numbers 1-9 are written out, numbers 10 or above are handwritten, and most importantly be consistent, if using one or the other:
ex: I bought 100 cats and two hundred dogs. Incorrect
I bought one hundred cats and two hundred dogs. Correct
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The general rule in formal writing (that which you do for school or work) is yes. All numbers that appear as two or fewer words should be written out.
The most formal kind of functional writing would be formal letters. This type of letter would be used in circumstances dealing with business matters or even when writing to a stranger.
2.3 could be written as two point three, or it could be written two and three tenths. What you choose would be a matter of convenience, depending on the length of the number. What you choose might also be a matter of formal style depending on the context of the writing.
The formal definition of a rational number is: Any number that can be written as a fraction with whole numbers on top and bottom. 3/4 certainly seems to fit that description.
You should write it in full in any formal or academic writing.