The most direct way of expressing 0.12 as a fraction is 12/100. However, most math teachers expect the fraction to be reduced to 3/25. There are, in fact, an infinite number of fractions that equal 0.12.
The fraction 120/1000 is equal to 0.12, as is 6/50, 24/200 and 1200/10000. In all these cases, the numerator and the denominator of 3/25 are multiplied by the same number. Since this is the same as multiplying by one, the value of the fraction is not changed. As long as the numerator and denominator are equal, any number can be used, so the possible ways of writing a fraction equal to 0.12 are infinite.