0.9 as a fraction is 9/10. To convert a decimal number into a fraction, we write the given number as the numerator and place 1 in the denominator right below the decimal point followed by the number of zeros required accordingly. Then, this fraction can be simplified. In this case, 0.9 has one number after the decimal, so we place 10 in the denominator and remove the decimal point. This gives us 9/10, which is already in its simplest form.